U.S. patent number 10,451,808 [Application Number 15/747,247] was granted by the patent office on 2019-10-22 for mems devices for smart lighting applications.
This patent grant is currently assigned to TRUSTEES OF BOSTON UNIVERSITY. The grantee listed for this patent is David J. Bishop, Matthias Imboden, Thomas Little, Jessica Morrison, TRUSTEES OF BOSTON UNIVERSITY. Invention is credited to David J. Bishop, Matthias Imboden, Thomas Little, Jessica Morrison.
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United States Patent |
10,451,808 |
Bishop , et al. |
October 22, 2019 |
MEMS devices for smart lighting applications
Abstract
The invention provides a device comprising a movable micromirror
adapted to receive light from one or more light source(s) and
manipulate the reflected light. The micromirror can be actuated
electrothermally. In particular, the micromirror is adapted to do
at least one of: (a) tipping along a first axis; (b) tilting along
a second axis; (c) changing focal length (i.e., varifocal mode);
and (d) elevating (i.e., piston mode). The invention also provides
a system comprising at least one device comprising a movable
micromirror and at least one light source. The invention can be
used in smart lighting applications.
Inventors: |
Bishop; David J. (Brookline,
MA), Little; Thomas (Newton, MA), Morrison; Jessica
(Lynn, MA), Imboden; Matthias (Brookline, MA) |
Applicant: |
Name |
City |
State |
Country |
Type |
TRUSTEES OF BOSTON UNIVERSITY
Bishop; David J.
Little; Thomas
Morrison; Jessica
Imboden; Matthias |
Boston
Brookline
Newtown
Lynn
Brookline |
MA
MA
MA
MA
MA |
US
US
US
US
US |
|
|
Assignee: |
TRUSTEES OF BOSTON UNIVERSITY
(Boston, MA)
|
Family
ID: |
57885259 |
Appl.
No.: |
15/747,247 |
Filed: |
July 22, 2016 |
PCT
Filed: |
July 22, 2016 |
PCT No.: |
PCT/US2016/043723 |
371(c)(1),(2),(4) Date: |
January 24, 2018 |
PCT
Pub. No.: |
WO2017/019557 |
PCT
Pub. Date: |
February 02, 2017 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20180231715 A1 |
Aug 16, 2018 |
|
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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62196791 |
Jul 24, 2015 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B
6/3584 (20130101); G02B 26/085 (20130101); G02B
26/0841 (20130101); G02B 6/3518 (20130101); G02B
6/357 (20130101); G02B 26/02 (20130101); G02B
26/06 (20130101) |
Current International
Class: |
G02B
6/12 (20060101); G02B 26/02 (20060101); G02B
6/35 (20060101); G02B 26/08 (20060101); G02B
26/06 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kim; Ellen E
Attorney, Agent or Firm: Nixon Peabody LLP
Government Interests
GOVERNMENT SUPPORT
This invention was made with Government Support under Contract No.
EEC0812056 awarded by the National Science Foundation. The
Government has certain rights in the invention.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a 35 U.S.C. .sctn. 371 National Phase Entry
Application of International Application No. PCT/US2016/043723,
filed Jul. 22, 2016, which designates the U.S. and which claims any
and all benefits as provided by law including benefit under 35
U.S.C. .sctn. 119(e) of the U.S. Provisional Application No.
62/196,791, filed Jul. 24, 2015, the contents of each of which are
incorporated herein by reference in their entireties.
Claims
What is claimed is:
1. A device comprising: a base substrate; a platform suspended over
the base substrate; a plurality of support elements supporting the
platform over the base substrate, wherein each support element has
a first end and a second end, wherein the first end of each support
element is mounted on the base substrate and connected to an
electrical source, and wherein the second end of each support
element is suspended over the base substrate, the plurality of
support elements being configured to cause the platform to move
relative to the base substrate; and a micromirror including a first
portion directly attached to the platform such that the first
portion cannot move relative to the platform and a second portion
that is not attached to the platform such that the second portion
can move relative to the platform, the micromirror including a
material that is configured to enable the second portion of the
micromirror to move relative to the platform responsive to an
input.
2. The device of claim 1, further comprising a plurality of spring
elements, wherein each spring element has a first end and a second
end, and wherein the first end of each spring element is connected
to the second end of each support element, and wherein the second
end of each spring element is connected to the platform.
3. The device of claim 1, wherein each support element includes a
multimorph material movable in response to an electrical
signal.
4. The device of claim 1, wherein the plurality of support elements
are configured to enable the platform to (i) tip along a first
axis; (ii) tilt along a second axis; (iii) elevate relative to the
base substrate, or (iv) any combination of (i)-(iii).
5. The device of claim 4, wherein the tipping of the platform along
the first axis, the tilting of the platform along the second axis,
or both has a range of -20 degrees to +20 degrees.
6. The device of claim 1, wherein the movement of the second
portion of the micromirror relative to the platform responsive to
the input causes a change in (i) a curvature of the micromirror,
(ii) a focal length of the micromirror, or (iii) both (i) and
(ii).
7. The device of claim 4, wherein the plurality of support elements
is configured to elevate the platform relative to the base
substrate by about 300 .mu.m.
8. The device of claim 3, wherein each support element comprises a
first layer comprising polysilicon, and a second layer comprising
gold, wherein the second layer is disposed on top of the first
layer.
9. The device of claim 8, further comprising an adhesion layer
comprising chromium or titanium disposed between the first layer
and the second layer.
10. The device of claim 2, wherein each of the plurality of spring
elements is stretchable.
11. The device of claim 2, wherein each of the plurality of spring
elements has a serpentine shape.
12. The device of claim 2, wherein each of the plurality of spring
elements comprises a semiconductor or metal.
13. The device of claim 12, wherein each of the plurality of spring
elements comprises polysilicon.
14. The device of claim 12, wherein each of the plurality of spring
elements comprises an alloy.
15. The device of claim 1, wherein the platform has a shape
selected from the group consisting of circular, oval, square,
rectangular, pentagonal, and hexagonal.
16. The device of claim 1, wherein the platform comprises a
semiconductor or metal.
17. The device of claim 16, wherein the platform comprises
polysilicon or single crystalline silicon.
18. The device of claim 1, wherein the first portion of the
micromirror includes a center of the micromirror and the second
portion surrounds the first portion.
19. The device of claim 1, wherein the micromirror comprises a
plurality of segments.
20. The device of claim 1, wherein the micromirror comprises a
first layer comprising polysilicon, and a second layer comprising
gold, wherein the second layer is disposed on top of the first
layer.
21. The device of claim 20, further comprising an adhesion layer
comprising chromium or titanium disposed between the first layer
and the second layer.
22. The device of claim 1, further comprising a heating element
positioned underneath the platform and adapted to heat up the
micromirror.
23. An optical network switch system comprising: a switch
backplane; a base substrate supported by the switch backplane; a
platform suspended over the base substrate; a plurality of support
elements supporting the platform over the base substrate, wherein
each support element has a first end and a second end, wherein the
first end of each support element is mounted on the base substrate
and connected to an electrical source, and wherein the second end
of each support element is suspended over the base substrate, the
plurality of support elements being configured to cause the
platform to move relative to the base substrate; a micromirror
including a first portion directly attached to the platform such
that the first portion cannot move relative to the platform and a
second portion that is not attached to the platform such that the
second portion can move relative to the platform, the micromirror
including a material that is configured to enable the second
portion of the micromirror to move relative to the platform
responsive to an input, the micromirror being positioned to receive
an optical signal from an input optical fiber and direct the
optical signal along a signal path towards an output optical
fiber.
24. The optical network switch of claim 23, wherein the plurality
of micromirror devices are arranged on the switch backplane in a
geometric, ordered, or random pattern.
25. The optical network switch of claim 23, further comprising a
switch controller configured to send signals to at least one of the
plurality of support elements of the first one of the plurality of
micromirror devices and actuate the micromirror of the first one of
the plurality of micromirror devices to direct the optical signal
along a predefined signal path.
26. A system comprising: a base substrate; a platform suspended
over the base substrate; a plurality of support elements supporting
the platform over the base substrate, wherein each support element
has a first end and a second end, wherein the first end of each
support element is mounted on the base substrate and connected to
an electrical source, and wherein the second end of each support
element is suspended over the base substrate, the plurality of
support elements being configured to cause the platform to move
relative to the base substrate; a micromirror including a first
portion directly attached to the platform such that the first
portion cannot move relative to the platform and a second portion
that is not attached to the platform such that the second portion
can move relative to the platform, the micromirror including a
material that is configured to enable the second portion of the
micromirror to move relative to the platform responsive to an
input; and at least one light source positioned to direct light
toward the micromirror, whereby application of an electrical signal
to at least one of the plurality of support elements moves the
platform to aid in controlling a reflection of light received from
the at least one light source.
27. The system of claim 26, further comprising a control unit
configured to control the application of the electrical signal to
at least one of the plurality of support elements.
28. The device of claim 1, wherein the material of the micromirror
comprises a multimorph material and the input is heat.
29. The device of claim 1, wherein the material of the micromirror
comprises a magnetic material and the input is a magnetic
field.
30. The device of claim 6, wherein the change in focal length is in
the range of -0.48 mm to 20.5 mm.
31. The device of claim 1, wherein the micromirror and the platform
are monolithic.
Description
TECHNICAL FIELD
The present invention relates to microelectromechanical systems
(MEMS) micromirrors and their applications in e.g., smart
lighting.
BACKGROUND
In almost all lighting applications, the distribution of light from
the source is fixed in time. Lights can be turned on and off and/or
dimmed but the distribution of the photons that do leave the source
is generally static. This means that much of the light is not going
into a useful direction. There are two limits to this problem. In
an incandescent bulb, the light is emitted almost uniformly in
space, essentially equally intense in all directions. Typical light
fixtures try to compensate for this by having mirrors or other
reflecting surfaces inside the luminaire to direct the light to
where it is wanted. Laser diodes are in the opposite limit. While
extremely efficient in terms of generating photons from electrons,
they emit light that is uni-directional, a tightly collimated beam.
For luminaires built with these sources, one has the opposite
problem, that of diffusing a collimated beam. This is typically
done with some kind of translucent element such as a plastic dome.
But in either solution the distributions do not change in time.
SUMMARY
The invention relates to devices and systems comprising
micromirrors that can be actuated through thermal, electrothermal,
or magnetic means. When light from a light source impinges on the
micromirror, the micromirror can be actuated to manipulate the
reflected light. For example, the micromirror can manipulate the
reflected light for visible light communication. In another
example, the micromirror can change the light distribution,
intensity, and/or color in a room.
In one aspect, the invention relates to a device comprising (i) a
base substrate; (ii) a platform suspended over the base substrate;
(iii) a plurality of support elements supporting the platform over
the base substrate, wherein each support element has a first end
and a second end, wherein the first end of each support element is
mounted on the base substrate and connected to an electrical
source, and wherein the second end of each support element is
suspended over the base substrate; and (iv) a micromirror at least
partially mounted on the platform, wherein the micromirror includes
(a) a multimorph material movable in response to a heat source, or
(b) a magnetic material movable in response to a magnetic field,
whereby application of an electrical signal to at least one of the
support elements actuates the micromirror.
In another aspect, the invention relates to a system comprising at
least one light source and at least one device described in the
above aspect, wherein the at least one device is adapted to reflect
light from the at least one light source and manipulate the
reflected light.
In accordance with some embodiments of the invention, the device
can further comprise a plurality of spring elements, wherein each
spring element has a first end and a second end, and wherein the
first end of each spring element is connected to the second end of
each support element, and wherein the second end of each spring
element is connected to the platform.
In accordance with some embodiments of the invention, each support
element includes a multimorph material movable in response to the
electrical signal from the electrical source.
In accordance with some embodiments of the invention, the
micromirror is adapted to do at least one of: (a) tipping along a
first axis when the electrical signal is applied to one of the
support elements; (b) tilting along a second axis when the
electrical signal is applied to one of the support elements; (c)
changing focal length when the electrical signal is applied between
any of two support elements; and (d) elevating with respect to the
base substrate when the electrical signal is applied to each of the
support elements.
In accordance with some embodiments of the invention, the tipping
and tilting can each be configured to have a range from -20 degrees
to +20 degrees, and larger or smaller ranges can be provided by
changing the geometrical structures of the device.
In accordance with some embodiments of the invention, the change in
focal length can have a range from -0.48 mm to 20.5 mm, and larger
or smaller ranges can be provided by changing the geometrical
structures of the device.
In accordance with some embodiments of the invention, the
micromirror elevating distance can range from about 100 .mu.m (or
less) to 1.0 mm (or more).
In accordance with some embodiments of the invention, the device
comprises at least 3 support elements and at least 3 spring
elements.
In accordance with some embodiments of the invention, the device
comprises 4 support elements and 4 spring elements.
In accordance with some embodiments of the invention, each support
element can comprise a first layer comprising polysilicon, and a
second layer comprising gold, wherein the second layer is disposed
on top of the first layer.
In accordance with some embodiments of the invention, each support
element can further comprise an adhesion layer including chromium
or titanium disposed between the first layer and the second
layer.
In accordance with some embodiments of the invention, each spring
element can be flexible.
In accordance with some embodiments of the invention, each spring
element can be stretchable.
In accordance with some embodiments of the invention, each spring
element can have a serpentine shape.
In accordance with some embodiments of the invention, each spring
element can comprise a semiconductor or metal.
In accordance with some embodiments of the invention, each spring
element can comprise polysilicon.
In accordance with some embodiments of the invention, each spring
element can comprise an alloy.
In accordance with some embodiments of the invention, the platform
has a shape selected from the group consisting of circular, oval,
square, rectangular, pentagonal, and hexagonal.
In accordance with some embodiments of the invention, the platform
can comprise a semiconductor or metal.
In accordance with some embodiments of the invention, the platform
can comprise polysilicon, or single crystalline silicon.
In accordance with some embodiments of the invention, the
micromirror can be center mounted on the platform.
In accordance with some embodiments of the invention, the
micromirror can comprise a plurality of segments.
In accordance with some embodiments of the invention, the
micromirror can comprise a first layer comprising polysilicon, and
a second layer comprising gold, wherein the second layer is
disposed on top of the first layer.
In accordance with some embodiments of the invention, the
micromirror can further comprise an adhesion layer including
chromium or titanium disposed between the first layer and the
second layer.
In accordance with some embodiments of the invention, the device
can further comprise a heating element positioned underneath the
platform and adapted to heat up the micromirror.
In accordance with some embodiments of the invention, the heating
element can comprise a laser chip or a heating coil.
In accordance with some embodiments of the invention, the at least
one light source is a light-emitting diode or laser.
In accordance with some embodiments of the invention, the at least
one light source is mounted on the micromirror.
In accordance with some embodiments of the invention, the at least
one light source is suspended over the micromirror.
In accordance with some embodiments of the invention, the system
can comprise an array of light sources. The array of light sources
can be arranged in a geometric, ordered, or random pattern.
In accordance with some embodiments of the invention, the system
can comprise an array of devices. The array of devices can be
arranged in a geometric, ordered, or random pattern.
In accordance with some embodiments of the invention, the device or
system can further comprise a control unit.
BRIEF DESCRIPTION OF THE DRAWINGS
The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawings will be provided by the Office upon
request and payment of the necessary fee.
FIG. 1 is a schematic of a cross section of a device 100 in
accordance with some embodiments of the invention.
FIG. 2 is a schematic of a support element 130 in accordance with
some embodiments of the invention.
FIG. 3 is a schematic of a micromirror 140 in accordance with some
embodiments of the invention.
FIG. 4A is a schematic of a system 400A in accordance with some
embodiments of the invention.
FIG. 4B is a schematic of a system 400B in accordance with some
embodiments of the invention.
FIG. 4C is a schematic of a system 400C in accordance with some
embodiments of the invention.
FIG. 4D is a schematic of a system 400D in accordance with some
embodiments of the invention.
FIG. 5 shows a top view of a system 500 in accordance with some
embodiments of the invention.
FIG. 6 is a schematic showing light communication without or with
steering capabilities.
FIG. 7A is a schematic showing the coupling of a LED or laser diode
chips to a MEMS micro-mirror to give one the ability to steer the
beam.
FIG. 7B shows a typical MEMS micromirror that can be aimed in any
direction within a cone of roughly +/-30 degrees.
FIG. 7C shows an example of a MEMS micromirror whose radius of
curvature can be tuned to change the focus.
FIG. 8 shows an example of some ray-tracing simulations on the
devices described herein to quantify the performance as well as how
one might use it in a room.
FIGS. 9A-9B are scanning electron microscope (SEM) images of the
micromirror at (FIG. 9A) 60.degree. and (FIG. 9B) from above
showing the initial rotation of the central plate and bias
values.
FIG. 10A is a diagram depicting bimorph deflection with increased
applied power. The sequence is depicted as the wedges are
heated.
FIG. 10B is a plot showing the average radial profile for various
applied powers as measured using an optical profiler.
FIGS. 10C-10D show the data from the optical profiler as the mirror
flattens depicted as a color gradient.
FIG. 11 is a graph showing curvature measured versus power based on
a spherical fit to the gold layer.
FIG. 12 is a set of graphs showing aberrations of the gold surface
of the mirror from the Zernike polynomials as measured in the Zygo
interferometer. The three most significant aberrations were
spherical (left), astigmatism (center), and coma (right).
FIG. 13 is a graph showing beam deflection angle plotting over the
bias voltage.
FIG. 14 is a graphical representation of the beam position on a
viewing screen. Each dotted circle represents a 5.degree. increment
from the center. The color of each point corresponds to the power
required to achieve that position.
FIG. 15 is a graph showing the height of the center of the
micromirror as all four bimorph legs are actuated
simultaneously.
FIG. 16A is a SEM image of micromirror device in accordance with
some embodiments of the invention.
FIG. 16B is an illustration of bimorph layers before and after
oxide etch and reduced curvature due to heating.
FIG. 17A is a SEM image of the mirror depicting the eight bimorph
wedges.
FIG. 17B is an illustration of the wedge shape as Joule heating in
the serpentine springs heats the platform and mirror.
FIG. 18 is a SEM image with an actuation overlay. Eight electrical
leads are used to control both deflection and variable focus.
Leakage current through the springs due to I.sub..+-..theta./.phi.
results in negligible power consumption and is not shown here.
FIGS. 19A-19B shows optical profiler measurements of (FIG. 19A) the
average radial profile from the center of the mirror, (FIG. 19B) a
surface plot for three actuation powers.
FIG. 20 shows curvature (top) and Zernike (bottom) versus power
based on a spherical fit to the optical profile of the gold layer.
The curvature is fit to .kappa.=(0.042 mm.sup.-1/mW)P.sub.f
mW-1.038 mm.sup.-1 with the exception of the last point where the
wedges come into contact with the platform.
FIG. 21A shows beam deflection angle plotted against actuation
power when P.sub.0=15 mW and when P.sub.0=30 mW.
FIG. 21B is a diagram illustrating the powers of the bimorph legs
during actuation.
FIG. 21C is a table showing the labels for the power provided to
each leg given as subscripts .+-..theta. and .+-..phi. as shown in
FIG. 18.
FIG. 22 is a plot showing the height above the substrate of the
center of the micromirror as all four bimorph legs are actuated at
the same power together. The red trace is a linear fit with a slope
of -2.68 .mu.m/mW.
FIGS. 23A-23B are resistance measurements and exponential decay
fits to R(t)=R.sub.0+R.sub.Iexp(-(t-t.sub.0)/.tau..sub.th) for a
current pulse through (FIG. 23A) a bimorph leg and (FIG. 23B) the
serpentine springs. The thermal time constants for the bimorph leg
and mirror respectively were 2.0 ms and 14.9 ms for a step up in
current and 2.5 ms and 11.7 ms for a step down in current.
FIGS. 24A-24B are graphs showing the power dissipated in the
bimorph leg based on the voltage ramp time.
FIGS. 24C-24D are the normalized deflection angles due to the
voltage ramps measured using a PSD for each of the ramp times
(inset graphs provide the actual voltage over time). The
normalizations are each offset to reduce overlap.
FIG. 25 shows fast Fourier transform (FFT) of the response when
heating (up) and cooling (down) for varying voltage ramp times. The
reduction in response of the first two resonant modes is clear when
the voltage ramp is provided over 5 ms when compared to a step
voltage.
FIGS. 26A-26B is a set of SEM images showing deflection along one
axis (tip or tilt) by a single support element.
FIGS. 26C-26D is a set of SEM images showing the varifocal mode of
the micromirror.
FIG. 27 is a set of optical images showing deflection of a light
beam by the micromirror.
FIG. 28 is a graph showing the optical deflection for diagonal
differential actuation (square), singular actuation (circle). Some
error bars are too small to be seen.
FIG. 29 is a graph showing the vertical deflection of the
micromirror as a function of total power dissipated.
FIG. 30 is an image of FEM simulations of the mirror showing the
difference in mechanical strain in the serpentine springs connected
to bimorph legs with a nonzero offset power V .sub.offset,b with an
overlay of the differential driving circuit.
FIG. 31 shows the measured frequency response data with zero offset
voltages is provided showing the corresponding mode shapes as
simulated in COMSOL.
FIGS. 32A-32B are SEM images showing the spring deformation (FIG.
32A) prior to actuation and (FIG. 32B) during actuation of the
bimorph.
FIG. 33 is a graph showing measured frequency response data for
varying V .sub.offset,a for a power of 0 mW, 12 mW and 24 mW in
each of the offset bimorphs. Insets are mode shapes simulated in
COMSOL.
FIG. 34 is a graph showing measured frequency response data for
varying V .sub.offset,b for a power of 0 mW, 12 mW and 24 mW in
each of the offset bimorphs. Insets are simulated mode shapes.
FIG. 35 is a circuit diagram for the device comprising a
micromirror, 4 multimorph support elements, and 4 spring elements
in accordance with some embodiments of the invention.
FIG. 36 is a circuit diagram for the device comprising a
micromirror, 4 multimorph support elements, and 4 spring elements
in accordance with some embodiments of the invention.
FIG. 37 shows a diagrammatic view of an optical network switch
according to some embodiments of the invention.
FIG. 38 shows a diagrammatic view of an optical network switch
according to some embodiments of the invention.
FIG. 39 shows a diagrammatic view of an optical network switch
according to some embodiments of the invention.
DETAILED DESCRIPTION
Aspects and embodiments of the invention relate to a movable
micromirror. The micromirror can be actuated thermally,
electrothermally, electrostatically and/or magnetically. In
accordance with some embodiments of the invention, the micromirror
is adapted to do at least one of: (a) tipping along a first axis;
(b) tilting along a second axis; (c) changing focal length (i.e.,
varifocal mode); and (d) elevating (i.e., piston mode). And thus
the micromirror can receive light from a nearby light source (e.g.,
LED, optical fiber or laser) and manipulate the reflected light for
a variety of applications. The movable micromirrors according to
embodiments of the invention can have four separately controllable
degrees of freedom in a single device: wide deflection angles along
two axes respectively, tunable focal length, and a piston mode.
FIG. 1 is a schematic of a cross section of a device 100 in
accordance with some embodiments of the invention. The device 100
can comprise a base substrate 110, a platform 120, a plurality of
support elements 130, and a micromirror 140. The plurality of
support elements can support the platform 120, thus suspending the
platform 120 by a predefined distance d.sub.0 over the base
substrate 110. The predefined distance d.sub.0 can be zero or
greater, e.g., 1 .mu.m to 800 .mu.m, 1 .mu.m to 700 .mu.m, 1 .mu.m
to 600 .mu.m, 1 .mu.m to 500 .mu.m, 1 .mu.m to 400 .mu.m, 1 .mu.m
to 300 .mu.m, or 1 .mu.m to 200 .mu.m. The micromirror 140 can be
at least partially mounted on the platform 120.
In accordance with some embodiments of the invention, each support
element 130 can comprise a first end 132 and a second end 134,
wherein the first end 132 can be mounted to the base substrate 110
and connected to an electrical source, and the second end 134 can
be suspended over the base substrate 110 and coupled to the
platform 120. The electrical source can be a current source, a
voltage source, or both. The electrical source can provide an
electrical signal of sufficient amount to actuate the support
elements 130 or micromirror 140. For example, the electrical source
can provide a current in the range of 10 mA to 1000 mA (e.g., 10 mA
to 800 mA, 10 mA to 500 mA, or 10 mA to 300 mA), or a voltage in
the range of 1 mV to 100 V (e.g., 1 mV to 50 V, 10 m V to 30 V, 10
mV to 10 V, or 10 mV to 500 mV). The number of support elements 130
can be 2, 3, 4, 5, 6, 7, or more.
Each support element 130 can include a multimorph material movable
in response to an electric signal (e.g., current or voltage) from
the electrical source or a temperature change. The multimorph
material can comprise at least two layers of material, each having
a different coefficients of thermal expansion. In some cases, these
layers can produce a displacement via thermal activation: a
temperature change can cause one layer to expand more than the
other and cause the support element 130 to bend or flex. In other
cases, these layers can produce a displacement via electrical
activation: electric field can cause one layer to extend and the
other layer to contract. In accordance with some embodiments of the
invention, the multimorph material of each support element 130 can
comprise a first layer comprising polysilicon, and a second layer
comprising gold, wherein the second layer is disposed on top of the
first layer. An adhesion layer can be disposed between the first
layer and the second layer. The use of an adhesion layer for metal
deposition on a semiconducting material is known in the art. The
adhesion layer can comprise chromium or titanium. The thickness of
the adhesion layer can be in the range of 1 nm to tens of
nanometers.
A multimorph or multimorph material can comprise two or more
materials that have different coefficients of thermal expansion.
Accordingly, a multimorph or multimorph material can comprise 2, 3,
4, 5, or more materials. The two or more materials in a multimorph
can be layered. In general, metals have larger coefficients of
thermal expansion than semiconductors. Plastics/polymers also have
larger coefficients of thermal expansion than semiconductors.
Non-limiting examples of multimorphs or multimorph materials
include any combination of semiconductors and metals, a combination
of silicon and silicon oxide, and a combination of semiconductors,
polymers, and plastics. In accordance with some embodiments of the
invention, the multimorph is a bimorph (i.e. two materials that
have different coefficients of thermal expansion).
The length of each support element 130 can be in the range of 1
.mu.m (or less) to 1500 .mu.m (or more), e.g., 10 .mu.m to 1250
.mu.m, 100 .mu.m to 1000 .mu.m, 250 .mu.m to 750 .mu.m. The length
of the support element 130 can be selected to accommodate the
desired tip and/or tilt angles as well as the desired elevation
range.
In accordance with some embodiments of the invention, each support
element 130 can further include a heating element adapted to heat
up the support element 130. The heating element can heat up the
support element 130 through photon-induced or electron-induced
heating. Exemplary heating elements include, but are not limited
to, a laser chip, a heating coil, an induction heater, and a
cathode ray tube. The magnitude of temperature change required to
actuate the support element 130 depends on the particular
multimorph material included in the support element 130. For a
multimorph material including gold and polysilicon, actuation can
occur when the temperature is raised to within a range from
100.degree. C. to 300.degree. C. (e.g., about 200.degree. C.). The
temperature can be lower or higher than 100.degree. C.-300.degree.
C. depending on the properties of the particular multimorph
materials included in the support element 130.
In accordance with some embodiments of the invention, each support
element 130 can also be actuated through an electrostatic control.
An electrostatic force can be generated between two conducting
plates (e.g., metal plates) upon the application of a voltage
difference (V) between the two plates. The magnitude of the
electrostatic force is known to be proportional to V.sup.2. For
example, a polysilicon pad can be positioned below each support
element 130 and directly on the base substrate 110. A potential
difference applied between the polysilicon pad and the support
element 130 can result in a force between the two structures and
the support element 130 can be pulled toward the polysilicon pad
attached to base the substrate 110.
In accordance with some embodiments of the invention, each support
element 130 can be actuated by electromagnetic control. An
electromagnetic force can be generated by a magnetic coil and a
magnet, such that the application of a current induces a magnetic
field that moves the coil relative to the magnet. For example, a
magnet can be mounted to the base substrate 110 and the platform
can be coupled to a coil positioned adjacent the magnet such that
energizing the coil causes the coil and the platform to move
vertically, away from the substrate 100. Optionally, a spring or
similar mechanism can be used to limit the coil movement and bias
or pull the platform back toward the substrate when the coil is not
active.
The support element 130 can have an inlet for an electrical current
to flow in, and an out let for the electrical current to flow out
after it passes through at least a portion of the support element
130. FIG. 2 shows the support element 130 in accordance with some
embodiments of the invention. As shown in FIG. 2, the support
element 130 can comprise a first end 210 and a second end 220. The
first end 210 can be mounted on the base substrate and the second
end 220 can be suspended over the base substrate. The first end 210
can further comprise a first electrode 212 and a second electrode
214. When connected to an electrical source, the first electrode
212 can serve as a current inlet and the second electrode 214 can
serve as a current outlet, or vice versa, allowing an electrical
current to flow through the support element 130. When the current
has sufficient magnitude, the current can actuate the support
element 130 by heating up the support element 130 above a threshold
temperature.
In accordance with some embodiments of the invention, the device
100 can further comprise a plurality of spring elements 150 (e.g.,
2, 3, 4, 5, 6, 7, or more). The plurality of spring elements 150
can be suspended over the base substrate 110. Each spring element
150 can comprise a first end 152 and a second end 154. The first
end 152 of each spring element 150 can be connected to the second
end 134 of each support element 130. The second end 154 of each
spring element 150 can be connected to the platform 120. Each
spring element 150 can be flexible and/or stretchable. The
plurality of spring elements 150 is adapted to bend, extend, or
twist to allow the platform 120 and the micromirror 140 disposed
thereon to tip or tilt to large angles. In addition, the plurality
of spring elements 150 can serve as heating elements for the
platform 120. To serve as heating elements, the plurality of spring
elements 150 can include a material having sufficient electrical
resistivity to produce heat through joule heating. Each spring
element 150 can have a shape that allows it to bend, extend, or
twist. In accordance with some embodiments of the invention, each
spring element 150 can include a serpentine or coil shaped portion.
In accordance with some embodiments of the invention, each spring
element 150 can include a fractal shaped or repeating geometric
shaped portion. Each spring element 150 can be composed of a
plurality of turns (e.g., 2, 3, 4, 5, 6, or more). The length of
each spring element 150 at a relaxed state can be 10 .mu.m-300
.mu.m, e.g., 10 .mu.m-200 .mu.m, or 10 .mu.m-100 .mu.m.
A variety of materials can be used for the spring elements 150.
Each spring element 150 can comprise a semiconductor (either doped
or undoped) or a metal. For a metal to work, it would typically
need to be an alloy because generally pure metals are soft because
the dislocation lines can move easily. However by alloying (adding
a few percent of something else) most metals can be made tough and
springy. For example, pure iron is soft but adding a few percent of
carbon can make it strong and tough. A few metals such as tungsten
can work as spring elements even as pure metals. In accordance with
some embodiments of the invention, each spring element 150 can
include polysilicon.
The platform 120 is adapted to be a support on which the
micromirror 140 rests. The platform 120 can be connected to the
micromirror 140, for example, at the center of the micromirror 140.
The connection can be formed as a result of an etching process,
such that the platform 120 and micromirror 140 can be connected by
the remaining material that is not etched away during the etching
process. Hence, actuation of the support elements 130 can move the
platform 120 and the micromirror 140 mounted thereon. In accordance
with some embodiments of the invention, the platform 120 and the
micromirror 140 can tip along a first axis. In accordance with some
embodiments of the invention, the platform 120 and the micromirror
140 can tilt along a second axis. In accordance with some
embodiments of the invention, the platform 120 and the micromirror
140 can elevate away from the base substrate 110.
In addition, the platform 120 is adapted to act as a thermal
contact for the micromirror 140 by transferring heat generated in
at least one of the plurality of spring elements 150 to the
micromirror 140. When a current of sufficient magnitude is flowing
through the plurality of spring elements 150, the plurality of
spring elements 150 can heat up, which in turn can heat up the
platform 120. The heated platform 120 can in turn heat up the
micromirror 140, causing the micromirror 140 to change its focal
length. In accordance with some embodiments of the invention, the
micromirror 140 can change from a flat configuration to a curved
configuration, or vice versa (e.g., FIG. 10A & FIGS. 26C-26D),
in response to a temperature change. The micromirror 140 can also
change from a convex configuration to a concave configuration, or
vice versa, in response to a temperature change.
Instead of using the spring elements 150 as a heat source, in
accordance with some embodiments of the invention, the device 100
can further comprise a heating element positioned on or below the
platform 120. The heating element can provide heat to the platform
120 and/or micromirror 140. Exemplary heating elements include, but
are not limited to, a laser chip, a heating coil, an induction
heater, and a cathode ray tube.
The platform 120 can be of any shape such as, without limitation,
circular, oval, square, rectangular, pentagonal, hexagonal, or
irregular shape. The platform 120 can also include at least one
cutout (e.g., 1, 2, 3, 4, 5, 6, or more) having any shape such as,
without limitation, circular, oval, square, rectangular,
pentagonal, hexagonal, or irregular shape. The thickness of the
platform 120 can be in the range of 100 nm to 10 .mu.m, such as 1
.mu.m to 5 .mu.m, or 1 .mu.m to 3 .mu.m. The platform 120 can
comprise a semiconductor or metal. In accordance with some
embodiments of the invention, the platform 120 can include
polysilicon. In accordance with some embodiments of the invention,
the platform 120 can include single crystalline silicon. In
accordance with some embodiments of the invention, the rim of the
platform 120 can be at least partially coated by a metal (e.g.,
gold, silver, copper, aluminum, or alloy) to aid in optical
reflectivity and prevent optically heating the platform
unintentionally. The platform 120 can have a thickness in the range
of about 0.1 .mu.m-10 .mu.m.
The micromirror 140 can have a reflective surface 142 facing away
from the base substrate 110. The reflective surface 142 can reflect
electromagnetic irradiation (e.g., light) impinging on the surface
142. The reflective surface 142 can include a reflective material
or structure or coating. The reflective material or structure can
be selected to have the desired reflectivity for the particular
wavelength of interest. Metals can be used as the reflective
material. The reflective structure can be a dielectric mirror which
can include alternating layers of dielectric materials. Dielectric
mirrors are useful because their optical characteristics can be
precisely engineered. In accordance with some embodiments of the
invention, the micromirror 140 can comprise a silicon layer and a
dielectric mirror disposed thereon, wherein the dielectric mirror
can comprise alternating layers of dielectric materials having
different coefficients of thermal expansion. Examples of dielectric
materials include, but are not limited to, porcelain (ceramic),
mica, glass, plastics, and the oxides of various metals. The
micromirror 140 can have any shape such as, without limitation,
circular, oval, square, rectangular, pentagonal, or hexagonal
shape.
In accordance with some embodiments of the invention, the
reflective surface 142 can further comprise at least one phosphor
material or fluorescence material. For example, the at least one
phosphor material can be used to convert light incident on the
micromirror to broad spectrum white light. In accordance with some
embodiments of the invention, the micromirror can redirect light
towards a phosphor material that is distant from the micromirror.
Examples of phosphor materials are shown in Table 1.
TABLE-US-00001 TABLE 1 Standard phosphor types Phosphor composition
color Zn.sub.2SiO.sub.4:Mn (Willemite) Green ZnS:Cu(Ag)(B*)
Blue-Green Zn.sub.8:BeSi.sub.5O.sub.19:Mn Yellow ZnS:Ag +
(Zn,Cd)S:Ag White ZnS:Ag + ZnS:Cu + Y.sub.2O.sub.2S:Eu White ZnO:Zn
Green (Zn,Cd)S:Cu Blue with Yellow persistence KCl green-absorbing
scotophor ZnS:Ag,Cl or ZnS:Zn Blue Zn(Mg)F.sub.2:Mn Orange ZnO:Zn
Blue-Green (KF,MgF.sub.2):Mn Orange-Yellow (Zn,Cd)S:Ag or
(Zn,Cd)S:Cu Yellow-green Y.sub.2O.sub.2S:Eu + Fe.sub.2O.sub.3 Red
ZnS:Cu,Al Green ZnS:Ag + Co-on-Al.sub.2O.sub.3 Blue
(KF,MgF.sub.2):Mn Orange (Zn,Cd)S:Cu,Cl Yellow ZnS:Cu or ZnS:Cu,Ag
Yellowish-green MgF.sub.2:Mn Orange (Zn,Mg)F.sub.2:Mn Orange-Yellow
Zn.sub.2SiO.sub.4:Mn,As Green ZnS:Ag + (Zn,Cd)S:Cu White
Gd.sub.2O.sub.2S:Tb Yellow-green Y.sub.2O.sub.2S:Tb White
Y.sub.3Al.sub.5O.sub.12:Ce Green Y.sub.2SiO.sub.5:Ce Blue
Y.sub.3Al.sub.5O.sub.12:Tb Yellow-green ZnS:Ag,Al Blue ZnS:Ag Blue
ZnS:Cu,Al or ZnS:Cu,Au,Al Green (Zn,Cd)S:Cu,Cl + (Zn,Cd)S:Ag,Cl
White Y.sub.2SiO.sub.5:Tb Green Y.sub.2OS:Tb Green
Y.sub.3(Al,Ga).sub.5O.sub.12:Ce Green
Y.sub.3(Al,Ga).sub.5O.sub.12:Tb Yellow-green InBO.sub.3:Tb
Yellow-green InBO.sub.3:Eu Yellow InBO.sub.3:Tb + InBO.sub.3:Eu
amber InBO.sub.3:Tb + InBO.sub.3:Eu + ZnS:Ag White
(Ba,Eu)Mg.sub.2Al.sub.16O.sub.27 Blue (Ce,Tb)MgAl.sub.11O.sub.19
Green BaMgAl.sub.10O.sub.17:Eu,Mn Blue
BaMg.sub.2Al.sub.16O.sub.27:Eu(II) Blue BaMgAl.sub.10O.sub.17:Eu,Mn
Blue-Green BaMg.sub.2Al.sub.16O.sub.27:Eu(II),Mn(II) Blue-Green
Ce.sub.0.67Tb.sub.0.33MgAl.sub.11O.sub.19:Ce,Tb Green
Zn.sub.2SiO.sub.4:Mn,Sb.sub.2O.sub.3 Green CaSiO.sub.3:Pb,Mn
Orange-Pink CaWO.sub.4 (Scheelite) Blue CaWO.sub.4:Pb Blue
MgWO.sub.4 Blue pale (Sr,Eu,Ba,Ca).sub.5(PO.sub.4).sub.3Cl Blue
Sr.sub.5Cl(PO.sub.4).sub.3:Eu(II) Blue
(Ca,Sr,Ba).sub.3(PO.sub.4).sub.2Cl.sub.2:Eu Blue
(Sr,Ca,Ba).sub.10(PO.sub.4).sub.6Cl.sub.2:Eu Blue
Sr.sub.2P.sub.2O.sub.7:Sn(II) Blue Sr.sub.6P.sub.5BO.sub.20:Eu
Blue-Green Ca.sub.5F(PO.sub.4).sub.3:Sb Blue
(Ba,Ti).sub.2P.sub.2O.sub.7:Ti Blue-Green
3Sr.sub.3(PO.sub.4).sub.2.cndot.SrF.sub.2:Sb,Mn Blue
Sr.sub.5F(PO.sub.4).sub.3:Sb,Mn Blue-Green
Sr.sub.5F(PO.sub.4).sub.3:Sb,Mn Blue-Green LaPO.sub.4:Ce,Tb Green
(La,Ce,Tb)PO.sub.4 Green (La,Ce,Tb)PO.sub.4:Ce,Tb Green
Ca.sub.3(PO.sub.4).sub.2.cndot.CaF.sub.2:Ce,Mn Yellow
(Ca,Zn,Mg).sub.3(PO.sub.4).sub.2:Sn Orange-Pink
(Zn,Sr).sub.3(PO.sub.4).sub.2:Mn Orange-Red
(Sr,Mg).sub.3(PO.sub.4).sub.2:Sn Orange-Pinkish White
(Sr,Mg).sub.3(PO.sub.4).sub.2:Sn(II) Orange-Red
Ca.sub.5F(PO.sub.4).sub.3:Sb,Mn 3800K
Ca.sub.5(F,Cl)(PO.sub.4).sub.3:Sb,Mn White-Cold/Warm
(Y,Eu).sub.2O.sub.3 Red Y.sub.2O.sub.3:Eu(III) Red
Mg.sub.4(F)GeO.sub.6:Mn Red Mg.sub.4(F)(Ge,Sn)O.sub.6:Mn Red
Y(P,V)O.sub.4:Eu Orange-Red YVO.sub.4:Eu Orange-Red
Y.sub.2O.sub.2S:Eu Red 3.5 MgO.cndot.0.5
MgF.sub.2.cndot.GeO.sub.2:Mn Red Mg.sub.5As.sub.2O.sub.11:Mn Red
SrAl.sub.2O.sub.7:Pb Ultraviolet LaMgAl.sub.11O.sub.19:Ce
Ultraviolet LaPO.sub.4:Ce Ultraviolet SrAl.sub.12O.sub.19:Ce
Ultraviolet BaSi.sub.2O.sub.5:Pb Ultraviolet
SrFB.sub.2O.sub.3:Eu(II) Ultraviolet SrB.sub.4O.sub.7:Eu
Ultraviolet Sr.sub.2MgSi.sub.2O.sub.7:Pb Ultraviolet
MgGa.sub.2O.sub.4:Mn(II) Blue-Green
FIG. 3 which shows a top view of the micromirror 140 in accordance
with some embodiments of the invention. The micromirror 140 can
comprise one or a plurality of segments 310 (e.g., 2, 3, 4, 5, 6,
7, 8, or more). By dividing the micromirror 140 into a plurality of
segments 310, the variable focus range of the micromirror 140 can
be increased as the mechanical stress at a curved configuration can
be reduced. The plurality of segments 310 can be substantially
disconnected from each other. For example, the plurality of
segments 310 can connect with each other at the center of the
micromirror 140. This segmentation permits each segment 310 of the
micromirror 140 to change from a flat configuration to a curved
configuration without incurring serious mechanical stress. In
accordance with some embodiments of the invention, each segment 310
can comprise a plurality of holes (e.g., 2, 3, 4, 5, 6, 7, 8, or
more). The plurality of holes can be used during the manufacturing
process to reduce etch time.
The focal length of the micromirror 140 can be changed thermally,
electrothermally, capacitively or magnetically. As described above,
having a heating element on or below the platform can change the
focal length of the micromirror 140 thermally. The amount of
temperature change required to actuate the micromirror 140 depends
on the particular multimorph material included in the micromirror
140. For a multimorph material including gold and polysilicon,
actuation can occur when the temperature is raised to within a
range from 100.degree. C. to 300.degree. C. (e.g., about
200.degree. C.). The temperature can be lower or higher than
100.degree. C.-300.degree. C. depending on the properties of
particular multimorph material included in the micromirror 140.
To change the focal length of the micromirror 140 electrothermally,
each segment 310 can include a multimorph material movable in
response to an electrical signal or a temperature change. In
accordance with some embodiments of the invention, the multimorph
material of each segment 310 can comprise a first layer including a
semiconductor material (e.g., polysilicon or single crystalline
silicon), and a second layer including a metal (e.g., gold, silver,
copper, aluminum, or any other reflecting metal), wherein the
second layer is disposed on top of the first layer. The second
layer can be reflective as a result of the metal. An adhesion layer
can be disposed between the first layer and the second layer. The
adhesion layer can comprise chromium or titanium. The thickness of
the adhesion layer can be in the range of 1 nm to tens of
nanometers.
To change the focal length of the micromirror 140 magnetically,
each segment 310 can include a magnetic material. Non-limiting
examples of magnetic materials include permanent magnetic
materials, ferromagnetic materials, ferrimagnetic materials,
superconducting materials and combinations thereof. The micromirror
140 can thus change its focal length in response to a magnetic
field (e.g., a current induced magnetic field such as a direct
current magnetic field).
Also shown in FIG. 3 are a first axis 320 and a second axis 322. In
accordance with some embodiments, upon actuation, the micromirror
140 can tip along the first axis 320. In accordance with some
embodiments, upon actuation, the micromirror 140 can tilt along the
second axis 322. The first axis 320 and the second axis 322 can be
orthogonal to each other.
It should be noted that the silicon mentioned throughout this
application can be either doped silicon or undoped silicon.
Accordingly, polysilicon can be either doped polysilicon or undoped
polysilicon; single crystalline silicon can be either doped single
crystalline silicon or undoped single crystalline silicon.
The electrical source(s) of the device 100 can be coupled to a
control unit. The control unit is adapted to control the amount of
electrical signal (e.g., voltage or current) applied to a
particular support element. The control unit can be operated
manually. The control unit can also include a program that permits
automated operation of the device 100. The control unit can include
a computer or microprocessor and associated memory (e.g., volatile
and/or non-volatile memory) for storing programs that can be used
to separately control the electrical signals to each support
element. The signals can be controlled to control their amplitude
(e.g., voltage and/or current) as well as use pulse width
modulation to control the duration of the signal. The pulse width
of the signal can be modulated to account for heat dissipation and
optimized to achieve response times on the order 1 ms or less.
The operation of the device can cause actuation of at least one of
the plurality of the support elements and/or at least one of the
plurality of segments of the micromirror. FIGS. 35-36 are circuit
diagrams for the device comprising a micromirror, four multimorph
support elements, and four spring elements in accordance with some
embodiments of the invention.
As shown in FIG. 35, each support element 130 can have a first
electrode 610 connected to the ground and a second electrode 612
connected to an adjustable power source (e.g., a rheostat 620 and a
power source 622). A voltage difference applied between the first
electrode 610 and second electrode 612 by the power source 622 can
result in an electrical current flowing through the support element
130, heating up the support element 130 due to Joule heating.
Tipping or tilting the platform and micromirror can occur when the
temperature of the support element 130 reaches or exceeds a
threshold temperature. A voltage difference can also be applied
between two support elements 130 by the power sources, each of
which can be connected through the platform 120 and spring elements
150 to one or more of the other support elements 130. An electrical
current can thus flow from one support element 130 to another,
heating up the spring elements 150 along the flow of the electrical
current. The heated spring elements 150 can heat up the platform
120 and micromirror 140, leading to a change in the focal length
and curvature of the micromirror 140.
As shown in FIG. 36, each support element can be connected to an
electrical source, allowing each support element to be actuated
independently; a separate electrical source (V.sub.m) can be used
to apply an electrical signal between any two support elements.
The movement of the support elements and micromirror can be
controlled by the application of an electrical signal (e.g.,
electrical current or potential) from the electrical source, and
can produce a variety of actuation modes for the micromirror. The
actuation modes for the micromirror include, but are not limited
to, tipping along a first axis, tilting along a second axis,
elevating the platform away from the base substrate (also referred
to as the piston mode), changing the micromirror curvature (also
referred to as the varifocal mode), and any combinations thereof.
Said tipping or tilting permits the micromirror to reflect an
incoming electromagnetic wave (e.g., visible light) towards a
desirable direction. Said tipping or tilting can be achieved by
actuating one of the support elements (e.g., applying an electrical
current to one of the support elements). When an electrical current
is applied to one of the support elements, the one of the support
elements can heat up due to Joule heating. The micromirror can tip
or tilt over a wide range. For example, the mechanical range of
said tipping or tilting can be from about -25.degree. to 25.degree.
or more, from about -20.degree. to 20.degree., from about
-15.degree. to 15.degree., or from about -10.degree. to 10.degree.
or less. The range of optical deflection is twice the mechanical
range. During said tipping or tilting, the focal length of the
micromirror can remain unchanged.
The platform and micromirror can be elevated (or dropped) with
respect to the base substrate when all the support elements are
actuated. Stated another way, the spacing between the platform and
the base substrate can be increased (or decreased) when all the
support elements are actuated. Said elevation can be achieved by
applying an electrical signal to all the support elements. The
maximum increase of said spacing can depend on the length,
composition, and thermal annealing procedure of the supporting
elements. In accordance with some embodiments of the invention, the
maximum increase of said spacing can be about 100 .mu.m or less. In
accordance with some embodiments of the invention, the maximum
increase of said spacing can be about 200 .mu.m or more. In
accordance with some embodiments of the invention, the maximum
increase of said spacing can be about 300 .mu.m or more. In
accordance with some embodiments of the invention, the maximum
increase of said spacing can be about 400 .mu.m or more. In
accordance with some embodiments of the invention, the maximum
increase of said spacing can be about 500 .mu.m or more. In
accordance with some embodiments of the invention, the maximum
increase of said spacing can be about 1000 .mu.m or more.
The curvature of the micromirror can also be changed through
electrothermal actuation. For example, the micromirror can be
changed from a curved configuration (e.g., concave) to a
substantially flat configuration, or vice versa. In another
example, the micromirror can be changed from concave to convex, or
vice versa. The change in micromirror curvature can be achieved by
applying an electrical signal between any of two support elements.
When at least one spring element is heated up, e.g., due to Joule
heating, a portion of the heat can be transferred to the platform
through heat transfer, thereby heating up the platform, which in
turn can heat up the micromirror. The micromirror can change its
curvature in response to the change in temperature. Due to the
difference in material composition, each spring element can have
larger electrical resistivity than each support element. This
impedance mismatch between the spring elements and support elements
allows for decoupling of the focal electrothermal actuation and
deflection electrothermal actuation. Each support element can have
a size sufficient to create a thermal barrier through which heat
due to actuation of the bimorph legs is impeded from heating up the
platform. The current required to heat up each spring element is
significantly smaller than that required to heat up each support
element.
The range of change in the focal length of the micromirror can
depend on factors such as thermal annealing procedure, composition
of the multimorph material, number of micromirror segments, and
placement of holes in the micromirror. In accordance with some
embodiments of the invention, the focal length of the micromirror
can change from about -0.48 mm to 20.5 mm.
In accordance with some embodiments of the invention, the device
can operate in a scanning mode, e.g., raster scanning. The
frequency of the scanning mode can depend on the response time of
the multimorph material used in the device.
The device described herein can be fabricated using standard
methods for making MEMS. These standard methods include processing
in a cleanroom. For example, the device can be fabricated using a
Multi-User MEMS Process (MUMPS) known as PolyMUMPs by MEMSCAP. See
D. Koester, A. Cowen, and R. Mahadevan, "PolyMUMPs design
handbook," at
http://www.memscap.com/products/mumps/polymumps/reference-material,
the contents of which are incorporated herein by reference. In one
embodiment, the fabrication can include three highly doped
polysilicon layers, two sacrificial oxide layers, and a gold layer
patterned using optical or electron-beam lithography.
One related aspect of the invention relates to a system comprising
at least one device as described herein having a micromirror and at
least one light source. The micromirror of the device can receive
light from the light source and manipulate the reflected light for
a variety of applications. In accordance with some embodiments of
the invention, the system can comprise an array of devices as
described herein (e.g., 2, 3, 4, 5, 6, 7, 8, or more) and one light
source. The array of devices can receive light from the light
source and manipulate the reflected light independently. In
accordance with some embodiments of the invention, the system can
comprise an array of devices as described herein (e.g., 2, 3, 4, 5,
6, 7, 8, or more) and an array of light sources (e.g., 2, 3, 4, 5,
6, 7, 8, or more). In accordance with some embodiments of the
invention, the system can comprise a single device as described
herein and an array of light sources (e.g., 2, 3, 4, 5, 6, 7, 8, or
more). The device can receive light from the array of light sources
simultaneously or sequentially. For example, color mixing can be
done in such configuration. The array of devices or light sources
can be arranged in a pattern. The pattern can be geometric, random
or ordered. Each of the light sources of the array can emit light
having the same or different wavelength(s).
The system can further comprise one or more optical element(s)
adapted to direct the light emitting from the light source to the
micromirror of the device. The one or more optical element(s) can
include static mirrors and/or refractive optics adapted to
structure the incident light prior to impinging on the micromirror.
The one or more optical element(s) can include molded plastic
parts.
The system can further comprise one or more optical element(s)
along the optical path of the reflected light, e.g., to enhance the
illumination properties. The one or more optical element(s) can
include diffusers, lenses, and mirrors.
In accordance with some embodiments of the invention, the light
source can be a light-emitting diode (LED). In accordance with some
embodiments of the invention, the light source can be a laser.
Pulse-width modulation techniques can be used to control the
intensity of the light source. In accordance with some embodiments
of the invention, the light source can be directed through an
optical fiber to the micromirror and/or the micromirror can direct
the light into an optical fiber for transmission to a remote
location. In accordance with some embodiments of the invention, the
system can further comprise a control unit coupled to the device
and adapted to actuate the device. The control unit can be operated
manually. The control unit can also include a program that permits
automated operation of the system.
FIG. 4A is a schematic of a system 400A in accordance with some
embodiments of the invention. The system 400A can comprise a device
410A having a micromirror 412A described herein and a light source
420A. The light source 420A can be mounted on the micromirror 412A.
For example, the light source 420A can be mounted at the center of
the micromirror 412A. The system 400A can further comprise at least
one optical element (e.g., lens) adapted to direct the light from
the light source 420A to the micromirror 412A. Depending on the
type of the light source and emission pattern of the light source,
the at least one optical element can vary.
FIG. 4B is a schematic of a system 400B in accordance with some
embodiments of the invention. The system 400B can comprise a device
410B having a micromirror 412B described herein and a light source
420B. The light source 420B can be mounted on a support 422B and
suspended over the micromirror 412B. While FIG. 4B shows the light
source 420B suspended over the center of the micromirror 412B, the
light source 420B can be positioned off-center relative to the
micromirror 412B. The system 400B can further comprise at least one
optical element (e.g., lens) adapted to direct the light from the
light source 420B to the micromirror 412B. Depending on the type of
the light source and emission pattern of the light source, the at
least one optical element can vary. The distance between the light
source 420B and the micromirror 412B can be adjusted to achieve the
desired outcome of reflection.
FIG. 4C is a schematic of a system 400C in accordance with some
embodiments of the invention. The system 400C can comprise a device
410C having a micromirror 412C described herein, a light source
420C, and an optical element 430C adapted to guide the light
emitting from the light source 420C towards the micromirror 412C.
In accordance with some embodiments of the invention, the optical
element 430C can comprise a waveguide (e.g., a plastic waveguide).
In accordance with some embodiments of the invention, the optical
element 430C can further comprise a series of lenses and filters.
The optical element 430C can be in the form of a single piece of
molded plastic. The system 400C can further comprise an enclosure
440C enclosing the device 410C, light source 420C, and optical
element 430C. Also shown in FIG. 4C is the electronic component
450C adapted to drive the device 410C and light source 420C. The
electronic component 450C can be positioned outside the enclosure
440C. The system 400C can further comprise a heat sink (not shown)
adapted to prevent the light source 420C from overheating.
FIG. 4D is a schematic of a system 400D in accordance with some
embodiments of the invention. The system 400D can comprise a device
410D having a micromirror 412D described herein, a light source
420D, an optical element 430D adapted to guide the light emitting
from the light source 420D towards the micromirror 412D, a lid
440D, and an extraction element 450D. The lid 440D can protect the
device 410D and light source 420D from being exposed to the dust.
The extraction element 450D can be a refractive lens, diffusive
lens, phosphor material, or any combinations thereof to act as a
medium of light extraction. The optical element 430D, lid 440D, and
extraction element 450D can be included in a single piece of molded
plastic.
The configurations in FIGS. 4C-4D have the advantage of having all
or most of the electrical wires on the same side of the system,
thereby simplifying electrical connections.
In accordance with some embodiments, the invention also provides a
system comprising (a) an array of devices as described herein, each
device having a micromirror, and (b) an array of light sources. The
system can comprise 2, 3, 4, 5, 6, 7, 8, 9, 10 or more devices. The
system can also comprise 2, 3, 4, 5, 6, 7, 8, 9, 10 or more light
sources. An exemplary system is schematically shown in FIG. 5. FIG.
5 shows a top down view of a system 500 in accordance with some
embodiments of the invention. The system 500 can comprise an array
of devices 510 and an array of light sources 520. The array of
devices 510 or light sources 520 can be arranged in a pattern. The
pattern can be geometric, random, or ordered. In accordance with
some embodiments of the invention, the system 500 can further
comprise a control unit coupled to the array of devices 510 and/or
array of light sources 520. The devices 510 can be controlled
independently or simultaneously by the control unit. The light
sources 520 can be controlled independently or simultaneously by
the control unit.
The devices and systems of the invention can be used in a variety
of light-steering applications. One application is in smart
lighting, a non-imaging system in which MEMS can be integrated with
solid state lighting to enable complete control over the flux and
the chromaticity of a lighting fixture. A subset of the smart
lighting applications includes MEMS in optical communication
systems. The devices and systems of the invention can eliminate the
need for additional optics to provide fully integrated directional
light. Beam steering can provide dynamic optical wireless
communications (OWC) for mobile systems while the tunable focus can
be used in location algorithms to pinpoint the location of a
receiver and focus on it. Micromirrors can provide a cheap and
effective method of introducing chip level control of illumination
in both space and time. The devices and systems of the invention,
by combing all the relevant degrees of freedom into a single, low
cost chip, can add this functionality to solid state lighting
systems in a practical way.
In accordance with some embodiments of the invention, the devices
and systems described herein can be used in visible light
communication. For example, light from a light source, which
carries data or information, can be steered to a user having a
receiver for receiving the reflected light, see, e.g., FIG. 6. By
incorporating the MEMS micromirror devices or systems described
herein, each receiver needs only one light source. Additionally,
the MEMS micromirror described herein can also be dynamically
focused so the beam can be both directed to the correct location
and focused or defocused to the correct size. One potential
application is automatic illumination in an area of a room where a
person has the receiver.
For visible light communication, light can be rendered using a
scanning mode in the system and incorporating a vector graphic
rendering scheme with one or more sources. This can allow vector
graphic representations within the illumination and can be used for
communications.
In accordance with some embodiments of the invention, the devices
and systems described herein can also be used to change at least
one characteristic of light in a room. For example, the devices and
systems described herein can be used to change at least one of:
spatial distribution, intensity, color, and hue. The change can be
programmed or initiated by a user. For example, the devices and
systems described herein can be used to paint a room with light,
wherein the intensity and color of the light can be tuned.
In accordance with some embodiments of the invention, the devices
and systems described herein can also be used for light harvesting.
For example, a percentage of the light from the light source can be
reflected directly from the substrate and the rest can be harvested
using the micromirror and dynamically controlled in both
illuminance and directivity.
In any of the applications of the devices and systems described
herein, the micromirrors can be dynamically actuated on time scales
faster than the persistence time of the human eye so that the light
does not appear to flicker to the human eye. For example, the
micromirrors can be dynamically actuated on a time scale of 5 ms or
less. The micromirrors can be dynamically actuated on time scales
sufficiently fast to eliminate speckle which can arise when lasers
are used. For example, the piston mode of the micromirrors can be
utilized to de-speckle a laser beam.
It should be understood that this invention is not limited to the
particular methodology, protocols, and reagents, etc., described
herein and as such may vary. The terminology used herein is for the
purpose of describing particular embodiments only, and is not
intended to limit the scope of the present invention, which is
defined solely by the claims.
As used herein and in the claims, the singular forms include the
plural reference and vice versa unless the context clearly
indicates otherwise. Other than in the operating examples, or where
otherwise indicated, all numbers expressing quantities of
ingredients or reaction conditions used herein should be understood
as modified in all instances by the term "about."
Although any known methods, devices, and materials may be used in
the practice or testing of the invention, the methods, devices, and
materials in this regard are described herein.
Definitions
Unless stated otherwise, or implicit from context, the following
terms and phrases include the meanings provided below. Unless
explicitly stated otherwise, or apparent from context, the terms
and phrases below do not exclude the meaning that the term or
phrase has acquired in the art to which it pertains. The
definitions are provided to aid in describing particular
embodiments, and are not intended to limit the claimed invention,
because the scope of the invention is limited only by the claims.
Further, unless otherwise required by context, singular terms shall
include pluralities and plural terms shall include the
singular.
As used herein, the term "comprising" or "comprises" is used in
reference to compositions, methods, and respective component(s)
thereof, that are useful to an embodiment, yet open to the
inclusion of unspecified elements, whether useful or not.
As used herein, the term "consisting essentially of" refers to
those elements required for a given embodiment. The term permits
the presence of elements that do not materially affect the basic
and novel or functional characteristic(s) of that embodiment of the
invention.
As used herein, the term "micromirror" refers to a small mirror,
typically having a dimension of less than 1 mm. In accordance with
some embodiments of the invention, the micromirror can be about 10
.mu.m to 2.0 millimeters or more across.
As used herein, the term "disposed on" refers to layers disposed
directly in contact with each other or indirectly by having
intervening layers therebetween, unless otherwise specifically
indicated.
Other than in the operating examples, or where otherwise indicated,
all numbers expressing quantities of ingredients or reaction
conditions used herein should be understood as modified in all
instances by the term "about." The term "about" when used in
connection with percentages may mean .+-.1% of the value being
referred to. For example, about 100 means from 99 to 101.
The singular terms "a," "an," and "the" include plural referents
unless context clearly indicates otherwise. Similarly, the word
"or" is intended to include "and" unless the context clearly
indicates otherwise.
Although methods and materials similar or equivalent to those
described herein can be used in the practice or testing of this
disclosure, suitable methods and materials are described below. The
term "comprises" means "includes." The abbreviation, "e.g." is
derived from the Latin exempli gratia, and is used herein to
indicate a non-limiting example. Thus, the abbreviation "e.g." is
synonymous with the term "for example."
Although preferred embodiments have been depicted and described in
detail herein, it will be apparent to those skilled in the relevant
art that various modifications, additions, substitutions, and the
like can be made without departing from the spirit of the invention
and these are therefore considered to be within the scope of the
invention as defined in the claims which follow. Further, to the
extent not already indicated, it will be understood by those of
ordinary skill in the art that any one of the various embodiments
herein described and illustrated can be further modified to
incorporate features shown in any of the other embodiments
disclosed herein.
All patents and other publications; including literature
references, issued patents, published patent applications, and
co-pending patent applications; cited throughout this application
are expressly incorporated herein by reference for the purpose of
describing and disclosing, for example, the methodologies described
in such publications that might be used in connection with the
technology described herein. These publications are provided solely
for their disclosure prior to the filing date of the present
application. Nothing in this regard should be construed as an
admission that the inventors are not entitled to antedate such
disclosure by virtue of prior invention or for any other reason.
All statements as to the date or representation as to the contents
of these documents is based on the information available to the
applicants and does not constitute any admission as to the
correctness of the dates or contents of these documents.
The description of embodiments of the disclosure is not intended to
be exhaustive or to limit the disclosure to the precise form
disclosed. While specific embodiments of, and examples for, the
disclosure are described herein for illustrative purposes, various
equivalent modifications are possible within the scope of the
disclosure, as those skilled in the relevant art will recognize.
For example, while method steps or functions can be presented in a
given order, alternative embodiments may perform functions in a
different order, or functions may be performed substantially
concurrently. The teachings of the disclosure provided herein can
be applied to other procedures or methods as appropriate. The
various embodiments described herein can be combined to provide
further embodiments. Aspects of the disclosure can be modified, if
necessary, to employ the compositions, functions and concepts of
the above references and application to provide yet further
embodiments of the disclosure.
Specific elements of any of the foregoing embodiments can be
combined or substituted for elements in other embodiments.
Furthermore, while advantages associated with certain embodiments
of the disclosure have been described in the context of these
embodiments, other embodiments may also exhibit such advantages,
and not all embodiments need necessarily exhibit such advantages to
fall within the scope of the disclosure.
EXAMPLES
The following examples illustrate some embodiments and aspects of
the invention. It will be apparent to those skilled in the relevant
art that various modifications, additions, substitutions, and the
like can be performed without altering the spirit or scope of the
invention, and such modifications and variations are encompassed
within the scope of the invention as defined in the claims which
follow. The technology described herein is further illustrated by
the following examples which in no way should be construed as being
further limiting.
Example 1
MEMS-Enhanced Smart Lighting
FIG. 7A is a schematic showing the coupling of a LED or laser diode
chips to a MEMS micro-mirror to give one the ability to steer the
beam. The MEMS micromirrors are used to dynamically redirect light
in any direction. FIG. 7B shows a typical MEMS micromirror that can
be aimed in any direction within a cone of roughly +/-30 degrees.
FIG. 7C shows an example of a MEMS micromirror whose radius of
curvature can be tuned to change the focus. Such mirrors are low
cost, easily fabricated in large numbers in a commercial fab,
reliable and can be actuated with only modest amounts of electrical
power.
The devices shown in the FIGS. 7B-C were built in the MEMSCAP
foundry using the PolyMUMPS process. This is a three layer process.
In research quantities, the processed silicon can cost as little as
.about.$1/mm.sup.2. In large volumes, the costs can drop by a
factor of five to ten. For a MEMS device, the cost is unrelated to
the complexity of the device. The costs are just proportional to
the surface area of the die whether there is a simple device on it
or a complicated one. Complexity is essentially free. The inventors
also have designs that combine both beam steering and the ability
to change the radius of curvature. By adjusting the currents
through the legs, the radius of curvature can be tuned as well as
the tilting angle. Such a device basically functions like a
miniature spotlight where the light can be both aimed and focused
where desired.
The device can be actuated via thermal bimorphs whose radius of
curvature is controlled by the amount of power dissipated in them.
Typical amounts of power needed are in the range of 50 mW per
support element. In addition to being low cost and low power, MEMS
devices have proven themselves to be quite reliable. The MEMS can
be designed such that the devices are operated within the linear
mechanical response regime so there is no failure due to material
fatigue. Cycling tests of similar devices into the tens of billions
of cycles have proven this. The other concern is mechanical shock
and vibration. Because of their small size, the mechanical resonant
frequencies are high where there is little mechanical noise.
Typical mechanical noise in a room falls off as 1/f and so by the
time one gets to a kHz and above, there is very little mechanical
noise to worry about. The same is true of shock.
Another advantage is speed. Because these devices are small, they
can move quickly. Typical response times are in the millisecond
range. Because of this, they can be used to move beams of light
around on time scales faster than the persistence of a human eye
and so dwell time in a particular direction can be used to adjust
both intensity and color if one has a number of these with
differing spectral characteristics. These devices allow one to
"paint" with light. FIG. 8 shows an example of some ray-tracing
simulations on the devices described herein to quantify the
performance as well as how one might use it in a room.
The MEMS micromirror devices described herein are low cost,
reliable, fast, have considerable functionality, easy to
manufacture and therefore are something that potentially could be
integrated into a solid state lighting solution in a practical
way.
Example 2
Electrothermally Actuated Micromirror with Integrated Varifocal
Capability
This example provides a micromirror design which incorporates
electrothermal deflection for both beam steering and a variable
focus. The focal length can be dynamically shifted between -0.7 mm
and +17.5 mm with less than 18 mW of power. Additionally, it has an
optical scanning range of up to .+-.30.degree. along both lateral
axes. Furthermore, by actuating all of the bimorph legs
simultaneously, the mirror can be actuated in piston mode providing
a fourth degree of freedom with a 150 um vertical range.
Varifocal Micromirror with Tip-Tilt-Piston Capabilities
The devices are fabricated using a Multi-User MEMS Process (MUMPs)
known as PolyMUMPs by MEMSCAP [MEMSCAP,
http://www.memscap.com/products/mumps/polymumps/reference-material].
The fabrication includes three highly doped polysilicon layers, two
sacrificial oxide layers, and a gold layer patterned using optical
lithography. The micromirror capabilities can be enabled using the
electrothermal properties of gold and polysilicon thin films. Using
thermal response allows for tunability of the curvature using Joule
heating [W. Liu and J. J. Talghader, Current-controlled curvature
of coated micromirrors, Opt. Lett. 28, 932-934 (2003)] while
maintaining large deflections. As the structure is heated, the gold
film experiences a greater expansion than the polysilicon resulting
in a change in curvature. In accordance with some embodiments of
the invention, the curvature of a bimorph structure is linearly
proportional to the change in temperature of the system.
Additionally, electrothermal driving depends inherently on Joule
heating which is linearly dependent on the dissipated electrical
power. Furthermore, the curvature of a bimorph actuator follows
linearly with the dissipated power.
As shown in FIGS. 9A and 9B, each of the four bimorph legs can be
connected (e.g., at the tip) to a serpentine (or coil) spring of
small cross sectional area. The springs can be connected to a
circular polysilicon plate, 400 um in diameter. As shown in FIG.
10A, a central cylinder of the polysilicon plate, consisting of
polysilicon and gold, acts as the thermal contact and mechanical
base of the mirror membrane. Eight thermal bimorph wedges 392 um in
length extend from the central cylinder. By dividing the mirror
into eight individual wedges, the bimorphs can deflect to a much
greater degree than would be possible with a solid plate. The
device can be tilted by applying a voltage V.sub..+-.x and/or
V.sub..+-.y, shown in FIGS. 9A-9B, such that only the biased
bimorph legs experience Joule heating and the mirror membrane
remains in its relaxed state. The piston mode is available by
actuating all four legs at the same power.
By applying a voltage difference between any of the two legs, shown
as V.sub.spr in FIG. 9B, the springs of small cross sectional
experience Joule heating and, in turn, heat the circular pad and
central cylinder. The central cylinder then heats the mirror
bimorph wedges, flattening them from their initial curved position
as depicted in FIG. 10A. This deflection is responsible for the
dynamic focal length adjustment and can be actuated independently
from beam steering.
An extensive study of the micromirror curvature and the resulting
focal range was conducted with a Zygo NewView 6300 Interferometer.
Diagrams depicting mirror curvature measurements using the
interferometer are shown in FIG. 10A. The curvature was measured by
fitting a sphere to the gold layer. Any spikes in the data from
reflection off of the circular heating pad below the bimorphs were
subtracted. Curvature measurements of the mirror membrane
demonstrate a wide focal range, when the legs are left unactuated,
from -0.7 0 m to +17.5 mm and can be flattened to within 230 nm.
The minimum fully decoupled focal length is limited by the residual
heat from actuating the bimorph legs. It is found to be -0.95 mm
(-0.53 mm .sup.-1 curvature) when all four legs are biased
regardless of the initial shape of the mirror. Additionally, the
curvature was the same to within less than 2.5% when two opposite
legs were at V.sub.spr (as shown in FIG. 9B) compared to when two
adjacent legs were offset at V.sub.spr. This implies that the
amount of heat migrating to the central cylinder used to heat the
wedges is the same regardless of which springs are used as the
source of heat. The central cylinder provides a unique advantage as
any uneven heating in the pad below the wedges is inconsequential
to their shape. Ultimately, the repeatability is limited by
hysteresis effects due to residual heat and thermal annealing
during actuation. A more detailed study on bimorph thermal cycling
has been performed by Gall et al. [K. Gall, M. L. Dunn, Y. Zhang,
and B. A. Corff, Thermal cycling response of layered
gold/polysilicon MEMS structures, Mech. Mater. 36, 45-55
(2004)].
FIG. 11 shows the curvature of two mirrors as a function of applied
electrical power. The curvature when the membrane is in a relaxed
state is dependent on the initial annealing procedure and can be
tuned to the desired minimum focal length by altering thermal
annealing times and temperatures Mirror 1 was measured in three
passes immediately following one another. The shift in curvature
from the first pass to the second is residual heat from cycling to
high power (completely flat) and immediately back to zero power.
The mirror returns to its original shape if one waits for the
device to cool completely. A more detailed study regarding the
relaxation time has not yet been conducted. The difference in
curvature between Mirror 1 (first pass) and Mirror 2 is due to
temperature variation in the initial annealing procedure.
The upper limit on curvature is determined by the close proximity
of circular plate beneath the wedges. This limit is apparent in
FIG. 11 from Mirror 2 as the mirror flattens and becomes
increasingly convex. Once the edges of the mirror touch the
circular plate, the bimorph wedges have a secondary place from
which they are heated. The most significant aberrations measured
using Zernike polynomials were spherical aberration, astigmatism
and coma shown in FIG. 12. The aberrations were all consistently
less than 800 .mu.m.
Electrothermal actuation can be used to produce large angle
mechanical deflections in the MEMS micromirrors. Vertical
(piston-mode) displacements of over 600 .mu.m [L. Wu and H. Xie, A
large vertical displacement electrothermal bimorph microactuator
with very small lateral shift, Sensors and Actuators A: Physical
145, 371-379 (2008)] have been achieved with minimal lateral
deflection. Additionally, angular displacements of over 30.degree.
have been obtained using electrothermal actuators [J. Sun, S. Guo,
L. Wu, L. Liu, S. Choe, B. S. Sorg, and H. Xie, 3D in vivo optical
coherence tomography based on a low-voltage, large-scan-range 2D
MEMS mirror, Optics express 18, 12065-12075 (2010)].
The optical deflection is shown in FIG. 13 for Mirror 1. Two
opposite bimorph legs were held at a constant 500 mV which pulled
the mirror to a baseline height lower than the maximum height. The
out of plane projection of the bimorphs forces a rotation of the
mirror structure. This rotation is annotated in FIGS. 9A-9B. An
asymmetry in the mechanical forces on the mirror shifts the pivot
point from center when actuated along one axis. The baseline height
is decreased in order to reduce rotation of the mirror while
actuating one leg independently from the other three and increase
the angular range. A third leg was held at 0 V while the bias for
the last leg was varied from 0 V to 550 mV (the variable deflection
leg).
The same angular range was demonstrated for each of the four
bimorph legs when acting as the variable deflection leg. The
optical deflection range is decreased to 26.degree. if the baseline
legs are at 0 V, such that the baseline height as the maximum
height of the mirror. FIG. 14 illustrates the position of the
shifted beam on a viewing screen above the micromirror. Each dotted
circle on the graph represents 5.degree. deflection from the
center. The lines closer to the horizontal and vertical axis are
the position as only one bimorph is actuated at a time up to
approximately 500 mV. The off axis lines are position measurements
when two of four bimorph legs are actuated simultaneously and held
at roughly the same power. In both cases, the maximum deflection is
between 24.degree. and 26.degree.. Thus, a non-zero constant power
is required to establish the baseline height in order to achieve
the 30.degree. deflection and reduce rotation of the mirror as it
is actuated. In accordance with some embodiments of the invention,
larger micromirror deflection angles (e.g., 90 degrees or more, but
only around one axis) can be achieved by configuring the system
with only 2 support elements to manipulate the micromirror support
platform.
Actuation of all four bimorph legs results in a piston-mode
vertical deflection. The total vertical deflection depends on the
initial curvature of the bimorph legs which can be tuned with an
initial rapid thermal annealing procedure. The displacement of two
mirrors, one of which is Mirror 1 from the data in previous
figures, is plotted in FIG. 15 against the power provided to all
four legs independently. Consequently, the total power is four
times larger than the value on the x-axis. As the mirror is pulled
toward the substrate, heat from the bimorph legs flows to the
mirror and the curvature is affected. However, the reduction in
focal range is limited as the curvature reaches a limit around
-0.53 mm.sup.-1 (-0.95 mm focal length) regardless of initial
curvature. The degradation of focal range due to actuating all four
bimorph legs is a worst case scenario for the mirror as the total
heat provided by tilting the mirror cannot exceed the heat provided
during vertical actuation unless intentionally heating the central
mirror.
A tip-tilt-piston micromirror with wide varifocal range according
to embodiments of the invention has been constructed and tested.
The focal length is tunable from -0.95 mm (-0.70 mm when bimorph
legs are not actuated) to +17.5 mm with <18 mW of electrical
power. The mirror can be deflected .+-.30.degree. symmetrically
along two axes. The maximum range requires the mirror to first be
pulled toward the substrate by two of the four bimorph legs.
Without the baseline offset from the actuation of two bimorph legs,
the optical deflection is between 24.degree. and 26.degree..
Vertical displacement can exceed 150 .mu.m, however, this sets a
limit on the minimum focal length to -0.95 mm. The integration of a
large range varifocal membrane and steering actuators has
tremendous implications in both optical systems in research and
innovative lighting products. The design simplifies what would
typically be a system of multiple optical components into a single
device, therefore reducing both cost and complexity.
Example 3
The micromirror device shown in FIG. 16A is fabricated using a
Multi-User MEMS Process (MUMPs) known as PolyMUMPs by MEMSCAP [18].
The fabrication includes three highly doped polysilicon layers, two
sacrificial oxide layers, and a gold layer patterned using optical
lithography. Residual compressive stresses in the polysilicon layer
combined with residual tensile stresses in the gold layer due to
the fabrication process provide a stress gradient along the
boundary of the gold and polysilicon layers. Upon release, a
bending strain due to the stress in the bimorph structures provides
an initial curvature as shown in FIG. 16B. Bimorph actuators rely
on a difference in coefficients of thermal expansion (CTE) of the
two layers. The CTE of the gold layer is greater than that of the
polysilicon resulting in a temperature dependent curvature [19],
.kappa., given as:
.kappa..ident..times..times..function..alpha..alpha..times..DELTA..times.-
.times..times..times..times..times..times..times..times..times..times..tim-
es..times. ##EQU00001##
where r is the temperature dependent radius of curvature, r.sub.0
is the initial radius of curvature at room temperature, t is the
sum of the individual layer thicknesses, t.sub.Au, and t.sub.Si,
.alpha..sub.Au and .alpha..sub.Si, are the temperature coefficients
of expansion for each layer, E.sub.Au, and E.sub.Si are the Young's
Moduli of each layer, and .DELTA.T is the temperature change.
Four bimorphs (e.g. support elements) can, for example, be
positioned tangential to the mirror (or mirror platform) acting as
the "legs" to raise and lower the mirror with respect to the
substrate. Side-angle views of the bimorph legs are shown in FIG.
16B. The curvature, and therefore the tip height, of each bimorph
leg can be finely tuned using Joule heating to deflect the mirror
toward or away from the substrate as is illustrated in FIGS. 16A
and 16B. To first order, the temperature change due to Joule
heating in a resistor is linearly proportional to the electrical
power, .DELTA.T.varies.I.sup.2R(T). Gold has a positive resistance
temperature coefficient and as a result, when current biasing the
system the power and temperature form a positive feedback loop
prior to thermal equilibrium. When voltage biasing the bimorph,
there is an initial power peak increasing the rate at which the
thermal equilibrium is established. Voltage biasing avoids the
runaway power increase possible when current biasing and is
typically the drive method of choice for systems with positive
thermal resistance coefficients [20]. However, as the resistance of
the bimorph legs is typically the same order of magnitude of the
resistance of the connected leads, a current bias provides better
stability and control when a four point probe is unavailable. In
accordance with some embodiments of the invention, the voltage can
be pulse width modulated to precisely control the joule heating and
precisely control the tip position of the bimorph support elements
relative to the substrate.
Each of the four bimorph legs can be connected, at the tip, to a
polysilicon serpentine spring, for example, having a 3.75
.mu.m.sup.2 cross sectional area, 160 .mu.m arm length and being
composed of 5 turns. These springs have a dual function: 1) they
enable the bending, extension and twisting required to allow the
mirror to be tilted to large angles and 2) as describe in detail
below, the springs serve as local heaters for the central plate.
While the serpentine springs are flexible enough to allow for large
angles, frequency response measurements have shown a vertical
(piston-mode) resonant frequency of .about.700 Hz. As an order of
magnitude approximation, the displacement of mirror under only
gravitational forces can be approximated by
.delta..apprxeq.g/.omega..sub.0.sup.2.apprxeq.500 nm, where g=10
m/s.sup.2. This demonstrates that while the springs provide enough
flexibility for angular deflection, the position of the mirror does
not significantly deviate in response to low frequency noise. The
springs can be connected to a circular polysilicon platform, 400
.mu.m in diameter. Eight polysilicon and gold bimorph wedges extend
from the center of the platform which functions as the thermal
contact for the mirror wedges to transfer heat to each wedge
causing the mirror wedges to change shape. The change in wedge
shape is depicted in FIG. 17B from an unactuated state (top) to a
heated state (bottom). By segmenting the mirror into eight
individual wedges (although more less wedges can used), the
bimorphs can deflect to a much greater degree than would be
possible with a solid membrane. The long silicon springs thermally
isolate the mirror from the bimorph legs.
The device can have at least three electrothermal actuation modes
which can be controlled independently. The scanning modes can be
achieved by applying a current I .sub..+-..theta. and/or I
.sub..+-..phi., as shown in FIG. 18. Much of the resulting current
is directed along the gold layer of the bimorph as a result of the
metal having much greater electrical conductivity than polysilicon.
The room temperature resistance of the serpentine springs is
R.sub.sp.apprxeq.10 k.OMEGA., 1500 times larger than the bimorph
leg resistance, R.sub.b.apprxeq.4.5.OMEGA.. The impedance mismatch
of the serpentine springs and bimorph legs allows for decoupling of
the focal electrothermal actuation and deflection electrothermal
actuation. Additionally, the 3.75 .mu.m.sup.2 cross sectional area
and significant length of the springs creates a thermal barrier
through which heat due to actuation of the bimorph legs is impeded.
Although the power required to heat the serpentine springs is
approximately the same as is required to heat the bimorphs, the
current required to heat the serpentine springs is two orders of
magnitude less than is required to heat the bimorphs. As a result,
the power dissipated in the bimorph legs due to the current
provided to heat the mirror is negligible compared to the power
required to heat the legs. To determine the leakage power through
the serpentine springs due to the actuation of a single bimorph,
the leakage current through each of the four springs can be
calculated using the potential at the tip of the actuated bimorph.
In this case, the power ratio dissipated in the springs with
respect to the bimorph scales as 9 R.sub.b/64 R.sub.sp for the
nearest spring and R.sub.b/64 R.sub.sp for the other three springs.
The piston mode is available by actuating all four legs at the same
power.
Heating the mirror wedges changes their curvature. This deflection
dictates the dynamic focal length as the radius of curvature is
twice the focal length, f=r/2. By introducing a current, I.sub.f,
between any of the two legs, as shown in FIG. 18, the serpentine
springs can be heated. The highly doped silicon also has a positive
resistance temperature coefficient resulting in an increase in
resistance to from R.sub.sp(T.sub.room).apprxeq.10 k.OMEGA. to
R.sub.sp(T.sub.hot).apprxeq.14 k.OMEGA.. The thermal energy in the
springs flows to the platform and the mirror bimorph wedges,
flattening them from their initial curved position. The resistance
of the bimorph legs increase from
R.sub.b(T.sub.room).apprxeq.4.5.OMEGA. to
R.sub.b(T.sub.hot).apprxeq.6.0.OMEGA. when heated. Thus, at any
temperature, the power dissipated in the springs due to leakage
current from the bimorphs is two orders of magnitude less than is
required to flatten the mirror.
The focal range can be characterized with an optical surface
profiler. FIGS. 19A-19B depicts mirror curvature measurements as
the mirror is actuated. FIG. 19A is a measurement of the average
radial height of the mirror relative to the center. For no
actuation, (P.sub.f=0 mW) a focal length of -0.48 mm is measured.
This can be tuned to 20.5 mm by providing 1.5 mA of current
(P.sub.f=27 mW) to the mirror such that each spring dissipates 6.75
mW.
The curvature, .kappa.=1/(2f), shown in FIG. 20 was measured by
fitting a sphere to the gold layer profile. The central thermal
contact provides a unique advantage as any temperature distribution
originating from the springs is inconsequential to the thermal
distribution on the wedges, as they are essentially heated from a
point source at the center. As a result, any asymmetry in power
dissipated between the springs does not degrade the optical
properties of the mirror. The curvature when the mirror is
unactuated is dependent on an initial thermal treatment and can be
tuned to obtain the desired minimum focal length by altering
annealing times and temperatures. The minimum focal length is also
dependent on the initial maximum actuation power, P.sub.f, which
determines the level of self-annealing. All subsequent actuations
are kept below the first maximum power to ensure
reproducibility.
The upper limit on curvature is determined by the 2 .mu.m proximity
of circular platform beneath the wedges. The most significant
aberrations, measured using Zernike polynomials, were spherical
aberration, astigmatism and coma and are shown in FIG. 20. The
spherical aberrations were consistently within .+-.300 nm, where
the astigmatism and coma aberrations fall in the 0-200 nm range,
until the last measurement where the curvature error was on the
same order as the aberrations (P.sub.f>26 mW) and the wedges
were in contact with the platform. This effect can be eliminated by
replacing the platform with a rim and radial attachments allowing
the wedges to deflect beyond the rim.
The reflectivity of the mirror is largely governed by the surface
area and scattering due to the release holes. The release holes
reduce the surface area by approximately 5% and the surface area of
the segment dividers reduces the effective mirror size by
approximately 9%. In all, the reflectivity of the mirror is 14%
less than a solid membrane before accounting for scattering and
diffraction losses. The surface areal losses can be reduced by
complete elimination of the release holes which would subsequently
increase the etch time. This would also reduce scattering and
diffraction losses. A more detailed study is required to understand
fully the overall optical losses as they are largely dependent on
the angle of incidence and the wavelength of the incident light
[21].
Electrothermal actuation can be used to provide large angle
mechanical deflections in the MEMS micromirror. Vertical (piston
mode) displacements of over 600 .mu.m [22] have been achieved with
minimal lateral deflection. Additionally, optical beam deflections
of over .+-.30.degree. to .+-.40.degree. can be obtained using
electrothermal actuators [23].
The optical deflection range of the MEMS present here is shown in
FIG. 21A. The out of plane projection of the bimorphs following the
oxide etch forces a rotation of the mirror about the vertical axis,
illustrated in FIG. 18. Thus, if a single bimorph is actuated the
mirror pivots such that the axis of rotation is not constant and
the symmetry of the four bimorphs is lost. To reduce rotation of
the mirror and platform, the baseline mirror height is decreased by
actuating two of the four bimorphs at an offset power, P.sub.0. The
other two bimorphs are actuated using a differential power based on
the initial current-voltage measurements. The power is produced
with a current bias such that
P.sub..theta.,.phi.=P.sub.1.+-.P.sub.Tilt, where P.sub.1=15 mW is
held constant at half the maximum power and P.sub.Tilt is varied
between 0 mW and 15 mW. The error in P.sub.Tilt is dominated by the
temperature dependence of the resistance and results in an
asymmetric power differential between legs. Two scenarios are shown
in FIG. 21A, the first with P.sub.0=15 mW yielding a total constant
power of 60 mW and the second with P.sub.0=30 mW yielding a
constant total power of 90 mW. When P.sub.0=15 mW, the angular
deflection is linearly proportional to the differential applied
power, P.sub.Tilt. Furthermore, the beam deflection along the
.theta. and .phi. axes are kept independent of one other, providing
a straightforward trigonometric relationship between the Cartesian
coordinates of the reflected spot on a screen and power provided to
the bimorph legs. Increasing the offset power to P.sub.0=30 mW
increases the maximum optical deflection. However, the angular
deflection at low P.sub.tilt is no longer linearly proportional to
P.sub.Tilt because the mirror pivot point is not held constant. The
asymmetry causes the axis of rotation to shift for each actuation
power such that the beam deflection does not follow a straight
line, convoluting the relationship between deflection angle
magnitude and beam direction.
Actuation of all four bimorph legs results in a piston mode
vertical deflection. The vertical displacement is plotted in FIG.
22 versus the total power provided to the four bimorph legs. As the
mirror is pulled toward the substrate, heat from the bimorph legs
flows to the mirror and the minimum focal length is increased
slightly to -0.87 mm. The reduction of focal range due to actuating
all four bimorph legs is a worst case scenario for the mirror as
the total heat needed for maximum tilt is lower than the heat
applied for full vertical actuation. However, it must be noted that
the piston and tip-tilt actuation modes are coupled. Actuating a
mirror vertically reduces the tip/tilt range, as now
P.sub..theta.,.phi., P.sub.0 and P.sub.1 are constrained.
Thermal cycling effects in the bimorphs limit curvature
reproducibility and contribute to a deflection spread of
approximately 3.degree. during the first actuation. As previously
mentioned, the first actuation of the mirror produces a nonlinear
curvature vs. power relationship while the bimorphs self-anneal. A
study on bimorph thermal cycling has been performed by Gall et al.
[24]. The thermal cycling study, however, does not include
temperature gradients due to electrothermal actuation. A more
detailed study of the mirror cycling is required to ensure long
term stability and deflection accuracy and is the focus of future
work. The reproducibility of the system can be greatly improved
upon by moving from the open-loop feedback control currently used
to closed-loop feedback system [25] with either a power or a
position sensitive PID loop.
The time dependent thermal response characterization of the system
can be performed by applying a current or voltage pulse to the
bimorph legs and the serpentine springs. For the legs, the power
resulting from the current pulse corresponded to an optical
deflection angle from approximately 10.degree. to 28.degree..
Likewise a current pulse resulting in a radius of curvature change
from -1 mm.sup.-1 to +0.05 mm.sup.-1 was used to measure the
thermal response time of the mirror. The thermal time constant,
.tau..sub.th, is determined by fitting the measured resistance to
the exponential function,
R(t)=R.sub.0+R.sub.1exp(-(t-t.sub.0)/.tau..sub.th), where R(t) is
the time dependent resistance, R.sub.0 is the resistance at low
power, R.sub.1 is the change is resistance, t is the time and
t.sub.0 is the time when the step in current occurred. The bimorph
leg time constants were measured to be 2.0 ms to heat the bimorph
legs, while the cooling time constant was 2.5 ms (data shown in
FIG. 23A). The serpentine springs and central plate/mirror have a
thermal time constant of 14.9 ms while heating and 11.7 ms while
cooling as determined from the data shown in FIG. 23B.
The difference in heating and cooling thermal time constants can be
attributed to changing material properties with changing
temperature [20]. Furthermore, when a current flows through the
legs power is generated along the entire structure, rapidly heating
the bimorph. When cooling all thermal power must flow to the base,
resulting in a faster heating time constant than cooling time
constant. This is the reverse for the mirror where heating is
locally restricted to the serpentine springs. The thermal energy
must then flow onto the mirror structure. The considerable surface
area allows for thermal cooling directly to the surrounding air.
Consequently, the mirror cools faster than it heats. A more
detailed measurement of the thermal distribution is required to
make assumptions regarding the temperature of the mirror wedges and
springs. The slight reduction in curvature when actuating the legs
indicates that the spring provides a considerable thermal barrier
between the legs and the platform.
The mechanical response times of the mirror deflection were
measured by detecting reflected light from the mirror with a
position sensitive detector (PSD). All of the mechanical response
measurements were driven with a voltage bias. The initial peak in
power with a voltage bias due to the positive temperature
coefficient of resistance results in shorter response times
compared to a current biased drive. To improve the accuracy of the
PSD measurements, the mirror was flattened to minimize the spot
size of the reflected light by keeping P.sub.f at a constant 26 mW.
FIGS. 24A-24D demonstrates the PSD measured response as a function
of time when a voltage ramp is used to actuate a single leg. Three
voltage drive schemes were implemented to reduce ringing and
overshoot. The voltage steps all correspond to a power modulation
of 0.02-27 mW (0-25.degree. optical deflection), corresponding to
an linear voltage ramp either increased from 10 mV to 400 mV or
decreased from 400 mV to 10 mV over 5 ms, 1 ms or less than 100
.mu.s (hereon referred to as a "step"). The mechanical rise time of
the mirror, defined by the time required to deflect from 10% to 90%
of the 25.degree. deflection, is given in Table 2 for each drive
scheme. Also included in Table 2 are the mechanical settling times
defined as the time required for the system to equilibrate to
within 1% of the final angle.
TABLE-US-00002 TABLE 2 Time Response of Micromirror Deflection Rise
Fall Angle Rise Angle Fall Voltage Ramp Rise Over- Settling Fall
Over- Settling Time Time shoot Time Time shoot Time 5 ms 5.2 ms
<1% 12 ms 4.1 ms <1% 7 ms 1 ms 4.4 ms 1.7% 18 ms 3.1 ms
<1% 10 ms Step (<0.1 ms) 4.8 ms 3.0% 21 ms 2.2 ms 1.9% 14
ms
A drastic reduction in ringing is clear for the responses of both
rising and falling actuation voltages in FIGS. 24C-24D when the
voltage is ramped over 5 ms compared to a step in bias. However,
the rise times for the step voltage and 5 ms ramp differ by less
than 0.5 ms indicating that the response time is limited by the
thermal, not mechanical characteristics of the system. In addition,
the overshoot in deflection angle is reduced to less than one
percent during both rise and fall powers for the 5 ms ramp.
Similarly, the settling time of the 5 ms ramp indicates very little
ringing. The fall time increased by a factor of two between a 5 ms
ramp and a step voltage. It is possible that this may be mitigated
by using more sophisticated driving techniques to allow the slope
of the voltage ramp to change over time so as to slow the
temperature change at the start and increase the rate of change in
voltage as the resistance plateaus.
The ringing for each drive scheme is most obvious when a Fourier
transform (FFT) is performed on the mechanical response. FIG. 25
illustrates the frequency response of the mirror for each voltage
ramp for both rise and fall of the system. The oscillation peaks
can be mapped to their respective resonance frequencies from both
measured data and simulated resonances. The first mode is a piston
mode with a frequency of .about.700 Hz and is only apparent during
a voltage rise. The second resonance is a torsional mode at
approximately 1 kHz. It is clear from the FFT that the torsional
mode shifts in frequency between the rise and fall data. Finite
element method (FEM) simulations show a frequency shift of
approximately 100-200 Hz as the temperature is increased, but a
more detailed study is required to fully understand the phenomenon.
Similarly, the 3 kHz resonance seen in the FFT data has been
observed during frequency sweeps with a lock-in amplifier and PSD
system, but the mode shape is not clear at this point in time. FEM
simulations show a higher order piston mode at .about.2.5 kHz and a
torsional mode at .about.2.8 kHz. The mechanical response of the
mirror wedges was not measured directly. However, it is assumed to
be limited thermally as length of the wedges requires the
mechanical resonance to be .about.25 times greater than the bimorph
legs, while the time to heat the springs is .about.2-3 times
longer. Thus, the thermal time constant governs the rate at which
the focal length can be changed.
A tip-tilt-piston micromirror with wide varifocal range has been
demonstrated. The focal length is tunable from -0.48 mm to +20.5 mm
with 27 mW of electrical power. The mirror can be deflected
.+-.40.degree. or more along two axes with 90 mW of total
electrical power. It should be noted that the deflection angle can
be increased by using longer support elements having larger
deflection ranges, potentially at the expense of response time and
power consumption. Vertical displacement of up to 300 .mu.m is
possible, however this sets a limit on the minimum focal length of
-0.87 mm and any vertical displacement puts a much more stringent
limit on the possible deflection angle. The system in accordance
with some embodiments of the invention also shows a response time
of approximately 5 ms for large angles and can be driven such that
any ringing is almost completely removed. The integration of a
large range varifocal mirror with steering actuators has
implications for both optical systems in research as well as
innovative dynamic lighting products. The design described herein
simplifies what would typically be a system of multiple optical
components into a single device, therefore reducing both cost and
complexity and significantly opening up the possible application
space for such mirrors.
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H. P. Herzig, O. Manzardo, C. R. Marxer, K. J. Weible, R.
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Frahm, M. Paczkowski, M. Haueis, R. Ryf, D. T. Neilson, S. Member,
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Example 4
FIGS. 26A-26D, 27-34 show a novel MEMS mirror which integrates
variable focus and dynamic beam steering, eliminating the need for
multiple microsystems. The mirror provides .+-.40.degree. optical
deflection and a variable focal length which can be tuned from
-0.48 mm to +20.5 mm. The mirror also can be actuated in a piston
mode over a 168 .mu.m range. The dynamics of the mirror were
studied and a methodology for tuning the resonant frequencies were
explored. The large optical deflection and variable focus ranges
provide unique advantages in Smart Lighting systems, where field of
view and dynamic optics are growing in demand due to the high
mobility of handheld receivers within the lighting field.
This example presents a dynamic study of the multi-degree of
freedom micromirror of the invention with large deflections and
variable focus. The rotational symmetry and identical spring
systems provide simple mapping from actuation power to angular
deflection. However, these two properties induce degenerate
resonance modes for tip and tilt scanning. Much has been done to
mode-match vibratory MEMS such as gyroscopes [S. Sung, W. T. Sung,
C. Kim, S. Yun, and Y. J. Lee, "On the mode-matched control of MEMS
vibratory gyroscope via phase-domain analysis and design,"
IEEE/ASME Trans. Mechatronics, vol. 14, No. 4, pp. 446-455, 2009].
In contrast, a system where degeneracies are suppressed is
desirable in many applications such as in large deflection systems.
For instance, the use of a scanning mode for color control in
standard illumination and projection systems is a use case scenario
where decoupled resonance modes are desired. Utilizing induced
mechanical stresses in a serpentine spring and bimorph system to
decouple degenerate resonant modes to increase the range of one
mode and decrease the energy lost to the degenerate mode is
discussed.
The micromirror can be fabricated by MEMSCAP using the PolyMUMPS
process [D. Koester, A. Cowen, and R. Mahadevan, "PolyMUMPs design
handbook," MEMSCAP Inc, 2003]. In this process, there is one
immobile polysilicon layer, two layers of polysilicon which can be
used as active mechanical layers and a gold layer atop the
uppermost polysilicon layer. The design presented in this paper
uses the residual stresses in the gold and top-most polysilicon
layer as a thermomechanical bimorph actuator in both the beam
steering and dynamic focus degrees of freedom. In both cases, a
combination of Joule heating and the difference in coefficients of
thermal expansion allow for large deflections of the steering
"legs" and the variable focus "wedges".
A voltage bias across the bimorph legs results in power dissipation
throughout the actuated leg. The thermal stresses induce a bending
moment along the bimorph leg resulting in a large vertical tip
displacement. The four serpentine springs extending out from the
mirror are vital to both the mechanical and the thermal properties
of the mirror. While the springs allow for an initial projection of
the mirror out of the plane upon release and provide the flexure
needed for large deflections of the mirror, they also act as heat
sources for the minor bimorph wedges. A large impedance mismatch
between the bimorph legs and the polysilicon serpentine springs
allows for full and independent control of angular deflection,
piston mode and tunable focus.
Previous designs incorporated 1000 .mu.m long (and longer) bimorph
legs in an effort to improve angular deflection. The length of the
bimorph legs largely governs the amount of lateral motion of the
tip of the bimorph demonstrated in FIGS. 26A and 26B. While
providing the required height, the added length decreases
efficiency as the first few milliwatts of power contribute mostly
to lateral motion. Additionally, gold on polysilicon bimorphs
become unstable as a threshold temperature is reached [K. Gall, M.
L. Dunn, Y. Zhang, and B. a. Corff, "Thermal cycling response of
layered gold/polysilicon MEMS structures," Mech. Mater., vol. 36,
No. 1-2, pp. 45-55, January 2004], narrowing the window in which a
vertical displacement can be obtained for long beams. By reducing
the length of the bimorph legs from 1000 .mu.m to 600 .mu.m, the
lateral displacement is greatly reduced and the angular
displacement of the mirror was more easily obtained for this
geometry (e.g., using 4 support elements). Short bimorphs also
improve linearity in the tip/tilt angle versus dissipated
electrical power in the bimorph legs. FIGS. 26C and 26D demonstrate
the dynamic range of the focal length.
Prior to the angular measurements a current-voltage sweep was
conducted. The power-voltage relationship was then used to form an
open-loop driven system using a voltage look-up table. For each
static angular measurement, the mirror was voltage biased and the
current and voltage were recorded using a four-point probe.
The angular deflections for varying drive techniques were tested
for both devices and are shown in FIG. 28. The diagonal range is
measured using a power differential actuation where the four
bimorphs are actuated at 12 mW to begin. From point of symmetry (12
mW), two adjacent bimorphs are increased while the other two are
decreased until the power is 24 mW (or 0 mW). A greater error in
power is associated with the differential measurements because the
deflection associated with a 4 mW power differential symmetric
about 10 mW does not produce the same deflection for one symmetric
about 12 mW.
The vertical range of the mirror was measured using a Zygo optical
interferometer. The bimorph legs were attached in series and
current biased to ensure the same power dissipation in all four
bimorph legs. FIG. 29 shows the vertical deflection as a function
of the total power dissipated.
Frequency scans were obtained when applying a differential AC
voltage bias shown in FIG. 30 on two legs, while the other two legs
were biased with V .sub.offset,a(b). The deflections were measured
using a 2D position sensitive detector (PSD). The output of the PSD
was coupled to a lock-in amplifier and recorded during each
frequency sweep. Using the output from the PSD and the measured
position of the PSD relative to the mirror, the magnitude of the
angle was extrapolated and recorded during frequency sweep.
Finite element method (FEM) simulations of the device were
conducted using COMSOL Multiphysics. The simulation
eigenfrequencies of the mirror system were 1065 Hz for the piston
mode and 1762 Hz for the degenerate tip and tilt modes. The
measured resonances with zero offset and a V.sub.ac peak-to-peak
value of 3 mV are 1552 Hz for both tip and tilt modes depicted in
FIG. 31. The offset voltage for the frequency response shown in
FIG. 31 is V .sub.offset,(a,b)=0 mV. The piston mode resonance
position is located at 500 Hz with a higher order piston mode
located at 1 kHz. The simulations did not include the release holes
and slight variations in the gold/polysilicon ratio due to design
rules for PolyMUMPS.
Perfectly matched modes are useful for circular scanning but need
to be suppressed for raster scanning. In the context of
illumination and directional lighting, the most useful scenario is
a large angle raster scan. To provide this capability, a method was
constructed to either suppress the unwanted mode or to separate the
modes while maintaining large amplitudes. The principle behind the
decoupling is dynamically changing the strain in the serpentine
springs and bimorph legs. As shown in FIGS. 32A-32B, actuation of a
bimorph leg drastically alters the torsional and bending strain in
the corresponding serpentine spring. Simultaneous actuation of
opposite bimorph legs reduces the effective spring constant of the
mode associated with rotation of the affected springs. In addition,
the effective spring constant of the mode associated with rotation
about the unactuated serpentine springs is increased slightly. FIG.
30 shows the simulated deformation of the serpentine springs upon
applying a nonzero V .sub.offset,b while holding V .sub.offset, a=0
mV.
Similar methodologies have been proven effective in scanning
micromirrors for shifting a single mode by using a separate
actuator [R. Bauer, G. Brown, L. Li, and D. Uttamchandani, "A novel
continuously variable angular vertical comb-drive with application
in scanning micromirror," Proc. IEEE Int. Conf. Micro Electro Mech.
Syst., pp. 528-531, 2013], [J. I. Lee, P. Sunwoo, E. Youngkee, J.
Bongwon, and J. Kim, "Resonant frequency tuning of torsional
microscanner by mechanical restriction using MEMS actuator," Proc.
IEEE Int. Conf. Micro Electro Mech. Syst., pp. 164-167, 2009].
The frequency response for variations in V .sub.offset,a and V
.sub.offset,b, corresponding to dissipated power in each of the
offset bimorphs are shown in FIGS. 33-34. The determination of each
mode was performed by capturing deflections of incident light from
the mirror. The maximum response for the tilt mode (yellow) is
shifted to a smaller frequency as V .sub.offset,a increases. In
contrast, the tip mode (blue) increases. The increase in amplitude
as the offset voltage is increased is an artifact of the constant
voltage bias. A 3 mV peak-to-peak excitation corresponds to a
greater power dissipation amplitude for a greater offset voltage
and, consequently, a larger total energy in the system.
In contrast, if V .sub.offset,b is increased from 0 mV the tilt
mode (yellow) is shifted to higher frequencies while the tip mode
(blue) is shifted to lower frequencies. It is important to note the
change in piston mode response and higher order tip/tilt modes
(.about.2.6 kHz) in response to the offset variation. For both
offset variations, the overall mode separations are similar and are
summarized in Table 3.
TABLE-US-00003 TABLE 3 Summary mode separation due to mechanically
straining specific sets of serpentine springs V.sub.offset,a Mode
Or Offset Power Tip Mode Tilt Separation V.sub.offset,b (mW) (kHz)
Mode (kHz) (Hz) a 24 1.68 1.39 290 a 12 1.66 1.43 230 a, b 0 1.55
1.55 0 b 12 1.64 1.42 220 b 24 1.65 1.33 320
An improvement of the static angular range compared to previous
results was obtained by shortening the bimorph legs. While the
total range remained consistent with previous results, the full
range can be achieved without the use of differential power bias
with an offset, thus reducing the required power to 25 mW. However,
this produces a decrease in the vertical range to 168 .mu.m
compared to the previous 300 .mu.m vertical range.
The functionality of the serpentine springs can be expanded beyond
the mechanism for large deflections and used as a mechanical tool
to alter the response frequency of the resonant modes. By adding
strain to specified spring pairs, the degeneracies in the system
can be lifted. Additionally, the strain can be tuned to increase
the response for one of the degenerate modes while dampening the
other as demonstrated in FIG. 34.
Example 5
The micromirror systems shown and described herein can be used in
any device that incorporates a conventional MEMS mirror to provided
improved functionality and enhanced performance. For example, an
array of MEMS micromirror devices as described herein can be used
to construct an optical network switch, such as an Exa-scale or
Zetta-scale optical switch.
Examples of the basic construction of such a switch are shown in
FIGS. 37-39. Generally, the signal of an input optical fiber
(carrying data) can be focused onto a MEMS micro-mirror and the
angle of the micromirror can be controlled and changed to redirect
to optical signal into any one of an array of output optical
fibers. A controller can be used to control and change the angle of
the micromirror to change the path of the optical signal. The
controller can be used to provide either circuit switching or
packet switching. Circuit switching can be more easily configured
as the circuit (e.g., micromirror angles) can be defined and
constructed in advance of the optical signal transmission. Packet
switch can be provided with additional hardware that identifies
destination address or labels and dynamically configures the
micromirrors in real time to direct optical packets through the
switch. In accordance with some embodiments of the invention, the
switch can be configured to provide space division switching,
wavelength division switching, time division switching and
combination of space division switching, wavelength division
switching, and time division switching by using the controller to
control the angle of the micromirror to change the signal path of
an optical signal.
As shown in FIGS. 1-3, 4A-4B and 35-36, a MEMS micro-mirror
according to some embodiments of the invention can provide up to
four degrees of freedom in a single device (e.g., varifocal, piston
and tip/tilt). In static mode, the mirror can tip/tilt +/-40
degrees around two axes and access +/-55 degrees in a dynamic mode.
The scaling rules for large optical switches show that the number
of ports one can access are proportional to the square of the tilt
angles. Accordingly, a mirror like that shown in figures can be
used to build a system with (40/5.5)2.times.1296 or 68,500 ports.
This mirror can be used to construct large (10 k by 10 k or 50 k by
50 k arrays) cross-connects that enables exa-scale and zetta-scale
switch systems. In accordance with some embodiments of the
invention, a typical optical switch can be constructed using MEMS
micromirrors on a 1mm pitch. Using this scheme, a 10 k port device
switch could be constructed using a mirror (and optical fiber)
array that is 10 cm on a side or a 100 cm.sup.2, forming softball
sized switch fabric. Similarly, a zetta-scale switch could be
approximately 23 cm on a side or 500 cm.sup.2, forming soccer ball
sized switch fabric.
The thermal bimorph actuators used to move the mirror each require,
on average, 10 mW of power or roughly 50 mW per mirror. For an
exascale switch this gives roughly 500 watts of power or .about.5
W/cm2. Current microprocessors operate at roughly 150 W/cm2 and so
we are well within current design norms. For a zetta-scale switch,
the areal power density stays the same at .about.5 W/cm2 and the
total power increases to 2,500 Watts, well less than a current
generation electrical switch. The MEMS micromirror can be power
optimized. For example, the MEMS micromirror can be packaged in a
vacuum, different metals for the bimorph structure can be used and
the geometry of the bimorph can be further optimized to reduce the
power needed to actuate it (e.g., by as much as a factor of ten).
The bimorph requires a specified operating temperature and by
reducing its width or cooling atmosphere, one can reduce the power
needed to get to this temperature.
The MEMS micromirror devices are expected to cost about $1/mm.sup.2
in small quantities and $0.10/mm.sup.2 in volume. Fiber bundles and
lens arrays tend to have costs that don't scale with area and the
cost/port drops with increasing port size. The cost to build these
very high capacity switches is expected to be in the $10-100/port
range. Another cost consideration is drive electronics. The switch
according to some embodiments of the invention can operate on power
and can be driven using pulse width modulation techniques (PWM).
The PWM drive circuit can use a fixed voltage and use the timing of
fixed the voltage digital pulses to control the bimorphs (e.g.,
using one or more low cost FPGAs). It should be noted that the
costs of building an electronic switch with this kind of capacity
would be many orders of magnitude higher.
Currently networks are managed and groomed at the wavelength level,
roughly 100-400 Gb/sec. Wavelength add/drop multiplexors are
typically used for this task. As the overall scale of the data
capacity of a network increases, so does the scale of the smallest
tributaries that get actively managed. Typically one manages and
grooms a network at a scale that is 1000 to 10,000 times smaller
than the total aggregate capacity. If the granularity gets too
large, the network is not optimized and money is wasted. If the
granularity is too small, the cost overhead of dealing with the
small tributaries overwhelms any possible cost benefits. These two
limits set the natural scale for traffic grooming and management.
The large switches described herein allow optimization at scales as
small as 1/50 k of the total traffic, well within the kinds of
needed management granularity for any modern network. In accordance
with some embodiments of the invention, the switch can be a circuit
switch. In accordance with some embodiments of the invention, the
switch can be a packet switch. Today, most electronic switches are
packet switches that route individual packets to their
destinations. The energy costs of doing this today are nearly at
the breaking point. Bandwidth growth over the next ten to twenty
years will break this paradigm completely as aggregate capacities
will grow by a factor of 20-50 per decade. The energy scaling
considerations will, by necessity, likely drive a shift to high
capacity, optical circuit switches as described herein for the
highest levels of any network or data center.
Depending on the Q of the device, switching speeds for MEMS devices
can be in the range of one to hundreds of milliseconds. In
addition, engineered drive techniques can be used to improve these
times by as much as a factor of a thousand. Accordingly, it is
expected that a smaller switch configuration can use micromirror
devices that operate on the order of 10 ms and that larger switch
configurations can use micromirror devices that operate on the
order of 1 ms or less.
FIG. 37 shows a diagrammatic view of a basic switch 700 according
to some embodiments of the invention. The switch 700 can include
one or more input optical fibers 710 and one or more output optical
fibers 712 mounted to a fiber support guide 714 that aligns the
fibers, for example, in an array such that the optical signals are
aligned and directed at the individual micromirrors 730 of the
switch backplane 720. The micromirrors 730 can be mounted to and
arranged in an array (e.g., in a geometric, ordered, or random
pattern) on the substrate 721 of the backplane 720 to align with
optical fibers. The micromirrors can be electrically connected to
the switch controller 740 which produces the electrical signals
that are used to control the angular orientation of the
micromirrors 730 to control the path of optical signal. As shown in
FIG. 37, a first micromirror 730 can be used to direct the optical
signal (from an input fiber 710) to reflect off stationary mirror
750 toward a second micromirror 730 which can be used to direct the
optical signal to an output fiber 712. Optionally, a lens array 716
or other collimating components can be used to limit the dispersion
of the optical signal received from the input optical fiber 710 and
reduce optical losses. In accordance with some embodiments of the
invention, the beam focusing capabilities (e.g., tuning the radius
of curvature) of the micromirrors 730 eliminate the need for a
collimating components.
The switch controller 740 can include a computer processor and
associated memory (e.g., volatile and/or non-volatile memory) for
storing and executing programs that control the operation and
functionality of the switch.
FIG. 38 shows a diagrammatic view of a basic switch 800 according
to some embodiments of the invention. The switch 800 can include
one or more input optical fibers 810 mounted to a fiber support
guide 814 that aligns the fibers, for example, arranged in an array
(e.g., in a geometric, ordered, or random pattern), such that the
input optical signals are aligned and directed at the individual
micromirrors 832 of the input switch backplane 822 and one or more
output optical fibers 812 mounted to a fiber support guide 818 that
aligns the fibers, for example, arranged in an array (e.g., in a
geometric, ordered, or random pattern), such that the output
optical signals from the individual micromirrors 834 of the output
switch backplane 824 are aligned and directed at the output optical
fibers 812. The micromirrors 832 can be arranged in an array (e.g.,
in a geometric, ordered, or random pattern) on the switch backplane
822 and the micromirrors 834 can be arranged in an array (e.g., in
a geometric, ordered, or random pattern) on the switch backplane
824. As shown in FIG. 38, a micromirror 832 on the input switch
backplane 822 can be used to direct an optical signal from an
aligned input optical fiber 810 toward any one of the micromirrors
834 on the output switch backplane 824 and the micromirror 834 can
be used to direct the optical signal to any one of the output
optical fibers 812. The micromirrors 832, 834 can be electrically
connected to the switch controller 840 which produces the
electrical signals that are used to control the angular orientation
of the micromirrors 832, 834 to control the path of optical signals
through the switch 800.
The switch controller 840 can include a computer processor and
associated memory (e.g., volatile and/or non-volatile memory) for
storing and executing programs that control the operation and
functionality of the switch.
FIG. 39 shows a diagrammatic view of a basic switch 900, similar to
switch 800, according to some embodiments of the invention. The
switch 900 can include one or more input optical fibers 910 mounted
to a fiber support guide 914 that aligns the fibers, for example,
arranged in an array (e.g., in a geometric, ordered, or random
pattern), such that the input optical signals are aligned and
directed at the individual micromirrors 932 of the input switch
backplane 922 and one or more output optical fibers 912 mounted to
a fiber support guide 918 that aligns the fibers, for example,
arranged in an array (e.g., in a geometric, ordered, or random
pattern), such that the output optical signals from the individual
micromirrors 934 of the output switch backplane 924 are aligned and
directed at the output optical fibers 912. The micromirrors 932 can
be arranged in an array (e.g., in a geometric, ordered, or random
pattern) on the switch backplane 922 and the micromirrors 934 can
be arranged in an array (e.g., in a geometric, ordered, or random
pattern) on the switch backplane 924. As shown in FIG. 39, a
micromirror 932 on the input switch backplane 922 can be used to
direct an optical signal from an aligned input optical fiber 910
toward any one of the micromirrors 934 on the output switch
backplane 924 and the micromirror 934 can be used to direct the
optical signal to any one of the output optical fibers 912. The
micromirrors 932, 934 can be electrically connected to the switch
controller 940 which produces the electrical signals that are used
to control the angular orientation of the micromirrors 932, 934 to
control the path of optical signals through the switch 900. Switch
900 differs from switch 800 in that the length of the optical path
of from any of the input optical fibers 910 to one of the output
optical fibers 912 can vary widely resulting in wide signal
variation. This can be mitigated using the focusing capabilities of
the micromirrors to tune or focus the optical signals on longer
paths to minimize the signal variation. In accordance with some
embodiments of the invention, after the controller 940 defines the
signal path, the controller 940 can determine a measure of the
length of the signal (e.g., based on the known geometric
configuration of the optical fibers and the mirrors) can tune the
radius of curvature of the micromirrors to further minimize signal
variation.
Other embodiments are within the scope and spirit of the invention.
For example, due to the nature of software, functions described
above can be implemented using software, hardware, firmware,
hardwiring, or combinations of any of these. Features implementing
functions may also be physically located at various positions,
including being distributed such that portions of functions are
implemented at different physical locations.
Further, while the description above refers to the invention, the
description may include more than one invention.
* * * * *
References